How to parent a child with ADHD?

I have a question. My husband and I do foster care and have had the same kids for 3 years. The youngest was diagnosed as severe ADHD. He has severe behaviors at school home and even on the bus. We do huge rewards like last week we went and done a day at the park, we went to Dollywood l, we went to the Aquarium we went to arcades we do things like this all the time and after rewards he does the same things. We have taken toys we have taken play time we have taken free time we have done timeout and nothing works. Any advice from anyone?