I would say it is highly inappropriate and unfortunately part of the autism and sensory stuff. I would try to find/make some things he is allowed to have in the tub, provided he cleans it out AND he has a fund set aside to pay a plumber if he sends it down the drain. At 13 that is a reasonable expectation if he wants to do this, in my opinion. I would probably call a plumber and find out what the minimum cost of a visit is, and the hourly rate, and set it above that. he can do chores or use birthday/christmas money if he has or gets that. Or he can go beg a grandparent for it, whatever as long as HE is the one who comes up with the money and he is not stealing or doing anything with other people's stuff to get it.
People on the autism spectrum often have very severe sensory issues and they can lead to really strange behavior. Some of it can be incredibly hard to handle as a parent. Wiz used to take a pound of raw burger out of the freezer and try to "cook" it with spices and eat it. made himself very very sick twice. He stopped because I made him pay the entire cost of teh doctor visit and the copay for the medications. wouldn't let him just pay the copay for the doctor visit because it was not high enough to really make an impact on him. yes, maybe it was "mean" or overly harsh, but it was that or endure endless rounds of this stupid behavior. I wasn't willing to risk his life to an e coli infection from eating raw meat that he let sit out for several days in his room - because he was still eating it when it had been out of the freezer for over two days. I have a friend who has a child with one kidney - that is not his own, and a very short digestive tract and most likely a very short life span (last I heard docs said it would be a miracle if he saw thirty) because he got e coli at age 2. I don't play with that. And money was the one thing that hit Wiz where it really hurt and couldn't be tolerated.
If he MUST have something to play with in the tub, let him fill a sock or the foot of a pair of pantyhose iwth oatmeal. Tie it shut and let him squish it around in the water, rub all over him, etc..... It is the same as those expensive aveeno bath packets, very good for the skin, not terrible to clean out (as long as you do not open the sock), and not bad for the drain. Now it will cause drain problems if he opens it and pours the mushy oats out, but if he will leave it in the sock it idsn't a problem. I do this with sunburns, rashes, etc.... because we could never afford the aveeno stuff. Well, we could but paying five bucks or more for eight packets of instant oatmeal ground up in a blender seems really stupid to me, so I put old fashioned or quick cook oats from the canister into a sock and either let the kid squish it or I tie it onto the faucet so the running water goes through it (tie it shut and then tie to the faucet). It works very well and is easier on the plumbing than the aveeno according to the plumbers I know.
Also get yourself a copy of The Out of Sync Child Has Fun by Kranowitz. It is PACKED with sensory activities and they always sparked even more ideas for my family. It is the most used book of games/activities that we have, lol, cause we ALL like the activities. We don't all like the same ones, but we all like some of them.
Also look for various books of kids' experiments - there are lots of them out there. Plus maybe get him into making soaps, lotions, bath salts, etc.... Let him look up what would be good for his skin and then figure out a way to make them. I have found ways to make a LOT of expensive spa products. I LOVE the sugar scrubs that so many places sell but I HATE the price tags. I don't much like the salt scrubs because I have so many sore spots from a skin problem and they sting. To make a sugar scrub you put sugar (granulated or the larger crystal turbinado - aka sugar in the raw - or a mix of the two) in a container and pour vegetable oil and whatever scent you want to cover. Stir together and add enough oil to make a loose paste. Can keep in the fridge for about six months and on the shelf for at least 3 months. I also make a cream for my psoriasis that turns out to be amazing on wrinkles. I get most of the ingredients from ebay for the best prices. I melt shea butter over hot water, stir in macadamia nut oil, vit E oil and pumpkin seed oil and melted beeswax (a small amount to thicken it). After it is stirred together well I put the container into an ice water bath and stir until it is cool with a whisk. Then it goes into little jars. My aunt in OH and her lady friends clamor for this for their wrinkles. I have put various other things in it, depending on what I have on hand and want at the time. I do NOT use coconut oil because it is actually very hard on the skin. Cocoa butter is not, but the coconut oil everyone raves about is really hard on your skin.
Anyway, my search for this cream started wehn I was allergic to most everything any doctor suggested for my skin problems. I was tired of paying a fortune for a little bottle of stuff and then having to give it away because it made things worse. I started reading and playing around with stuff.
Your son can learn to do this stuff. He can figure out ways to indulge this sensory need with-o ruining your plumbing or your tub. It will take some help on your part to keep him focused, and I would lock the bathroom door so that he can be searched before he goes in.
another thing he may like is to mix cornstarch and water in roughly equal amounts and put it into a big pan. It should be just enough to cover the bottom. I use a nine by thirteen pan, but you could use a cookie sheet if it has sides that come up a bit. Let him add food color if he wants (mix it with the water first). It is a fun mixture to play with, If you run a finger through it, the goop will move like a liquid, but if you hit it sharply it will break like a solid. My aunt's kids spent hours playing iwth this, as did my younger two. Wiz was grossed out by it though, part of his sensory stuff. He also hated finger paint of any kind, even pudding paint. Only kid in his preschool to refuse to join in when they did pudding paint (use vanilla pudding and food color as paint).
I can probably find titles if you want/need specific books on experiments and making your own soaps and things. He could even start with the melt and pour soaps and add various things to them to make them different shapes and testures.
I would insist that NO rice go into the bathroom. rice can swell to up to twenty times its dried size and is the single hardest thing in your home for your pipes. It is even worse for the bathroom drains than the kitchen ones because the bathroom drains tend to have hair in them in some amount and it catches the rice so it cannot move past.
