I am not sure of your son's sensory issues (that is, what kinds of clothes he will tolerate etc.) so I will just offer one idea that you may have already tried. If he will wear one piece pj's (at least at night) with NO feeties in them so you can put them on BACKWARD and the zipper goes up the back (you may even need to put a safety pin under the zipper-pull so he can't undo it) then he cant get to the diaper as easily. If he is a kid who pulls his arms out of the clothes you need to make sure the wrist parts are tight enough that it is hard to get the hand out (again if he can't undo pins you could use those so after his hand is through one way you can tighten it). At least if he picks through the clothes that way the little gross gel things will stay mostly in teh clothes and you can undress him in the tub or something. Yeah, I've been there, though when he was much younger. If it is too hot for the winter ones, and if this works, it may be worth hiring someone to make summer weight pj's.
If it is happening during the day of course then there needs to be (sorry to say because I know how hard it is to have to do direct visual 24/7 care every single day) a lot of alternative sensory choices....cards on a ring to shake, squeeze balls to squish, whatever his reinforcement preference testing shows (when you present things and he picks them consistenly to find out what motivates him--parents often know what textures etc. they like without doing that of course!)
It could be that you could find someone to make a big kid kind of "onsie" that again actually has snaps that are placed up high in the back, making it harder to get to.
IF he is good at fasteners, and a great escape artist then heaven help you...SMILE! Of course I am just kidding. Ask any PT or Occupational Therapist (OT) you know because they often serve kids who have the same issues and have learned from clever parents.