If he really has ADHD then he wont go symptom free all summer long when he is with you. However it may be harder for u to see the symptoms because he is out of school & that is where most of the problems occur.
Strattera is the safest way to go. It is a non stimulant, unlike Ritalin, Adderol, & Concerta. Most children I know who have ADHD dont benifit from it though. Stimulants have an opposite effect on children with ADHD, which is why they r used to calm them down. Nothing can make u ADHD, so if he takes them & then stops it wont cause ADHD. ADHD is a chemical imbalance in the brain. However he should not stop taking them suddenly for any reason. If he going to quit then it needs to be under a doctors supervision & he will most likely slowly lower the dose to nothing. Suddenly stopping can cause withdrawl symptoms & can harm you, not only with ADHD medications but many others. I know some families who take their kids off their ADHD medicines during the summer & weekends, under doctors supervision of course. Maybe u could do it that way. If u dont see a need for him to be taking them all summer it would be the best bet. Medications like that lower their effect when taken consecutively, that is why most doctors will have parents pick a day for the child to skip, like a sunday, that way the medicine doesnt build up in the body causing a need for a regular upping of the dosage for it to work properly. I would talk to his mother & maybe u can all meet with the doctor together, that way u r being informed straight from the doctor & arent wondering if she knows what she is talking about.