My 6 year old son has behavioral issues. Advice please.?

My 6 year old son has been getting into lots of trouble in school. He is in kindergarten. He went to school previously pre-k 3 and 4. I didn't have too many issues with him there just an occasional not wanting to go back in after recess. Fast forward, he goes to kindergarten has 3 different teachers in 4 months because the permanent teacher was on maternity leave. He jumps on chairs, beats on kids, screams, yells, disrespects and the list goes on and on. My husband and I put him in an outpatient behavioral program. He goes there half a day, it's a regular clroom with other children with issues but very structured. One week after beginning this program he begins using really bad language and tells me the kids in that school use it so he wants to also. At home we have a behavioral chart for home and one for school. We have a reward box and once he earns X amount of stickers he gets something. This works wonders with my other children but with him it could go either way. I have tried everything I could think of. We have counseling, behavioral modification programs, (yes I'm a believer) and time-outs. I seriously need some advice on what else I can try. If anyone has had these issues with any of their kids...what worked?? Btw, he has ADHD and is on medication.