If the medications are just stimulants, then I wouldn't worry too much about not getting his medications at his dada's. I WOULD worry about the dad 'persuading' the son that the medications are bad for him; however, your son may be old enough to have worked out for himself, that he likes himself better (and other people like him better too) when he's on the medications. I think you need to talk to your son about this when he is back home, let him know that his doctor wants him on the medications, you want him on the medications, his teachers want him on the medications. The medications are supposed to help him concentrate better and be able to stay on task so he can get his work done. It's not his fault that he gets fidgetty without medications, it's just the way his brain works and the medications flick a switch in his brain that helps him control his own fidgets (mental and physical fidgets).
But his dad isn't used to seeing him able to stay focussed so well, it scared him a bit and worried him. His dad doesn't understand that THIS is normal, compared to bouncing off the walls. Is it possible that dad is a "bouncing off the walls" kind of guy himself, and felt validated at last by having a son just like him? And now you've medicated it all away.
I would tell the dad, "He is NOT a zombie, but yes, he is much quieter and more focused than you remember. He IS older, too - a bit more maturity makes a lot of difference, especially when he's concentrating on something. Why don't you go get a football or some other outdoor activity, and take him outside for a game? I'm sure you will find that when he gets outside into the fresh air, you will enjoy a game of something with him. Or take him to some sort of indoor sporting activity, such as a squash court, where you can both urn up some energy. I know he's still young but surely not too young to learn how to play squash?"
Or something like that. Because even on medications, ADHD boys will happily go play at something physical, the adrenalin helps them with any remaining fidgets and getting tired also helps.
You need his dad on side, so he won't undermine what you are trying to do. It is good he felt able to express his concerns to you - the boy clearly is very different form what he expected, and from what he remembered. He did a good thing in calling you to tell you. Don't be too hasty to 'punish' him by taking the boy back home again, or he will think twice before calling you with concerns again. next time his concerns could be more important.
If you feel you MUST go there, maybe go a bit early, prepared to go away for a while and have a coffee, so you can sit with ex and talk about his concerns. Ask him to point out what it is about difficult child's behaviour that seems zombie-ish. There IS a possibility his medication dosage is a bit too high - it can take a bit of adjusting, the dose level is not necessarily connected to symptom level. difficult child 3 is on four times the dose of his older sister.
Holidays and weekends CAN be used to give a medication break, although I've found with difficult child 3, he needs some level of medication or he doesn't cope. So we cut the dose back, if no schoolwork is needed to be done. Maybe on dad visits, difficult child could go onto a reduced dose? But you need the dad's observations because at some stage, they COULD be important. What he saw that made him say, "Zombie" may NOT be just that difficult child was sitting quietly working with his flute. it may have been something about his eye contact, or his mood, or some other facet. Maybe a tic that is so subtle and so gradual in onset that you may not have seen it.
People who don't see our kids often can get it wrong so easily. But their observations can also be useful, precisely because they DON'T see the kids often and therefore can pick up on changes which we haven't seen, since they had a gradual onset.
My view in summary - tact and diplomacy time, before charging in to the rescue. You have the opportunity to turn this fright into a positive.
Marg