Louise, in general kids with ADHD respond well to rewards. It helps to activate their attention circuits.
They do not respond well to sitting on the potty for long periods of time.
First, make sure that there is no medical reason for the toileting problems.
Make diaper changing matter-of-fact. Do not make it unpleasant, but give as little attention to it as possible. Make it quick; make it blah. You don't want your child to associate diaper changing with a wonderful, warm and close time with Mom.
WHENEVER, your child does something in a positive direction toward potty training, make it special. Give a lot of positive attention.
Find out how your child feels about his potty training. Have some conversations about this. Make the circumstances pleasant for the conversations.
Then, discuss rewards with your child. What does he like? Rewards are what HE wants, not what you think he SHOULD want.
Use small, but strongly desired reinforcers for small victories-such as, asking to go to the potty and making it there.
Use large, desperately desired reinforcers for large victories- such as, not having an accident in three days.
My son who has ADHD was 3 1/2 and still not potty trained. He didn't see any reason to change his lifestyle. He enjoyed the freedom that diapers gave him and the attention that he got when they were changed.
After a conversation, where he learned that the Mother's Day Out that he enjoyed would not let him enroll if he was not potty trained, he became interested. He loved M & Ms. We got started on training using them. He wanted a new puzzle. That was no problem after the first day. We went and picked the puzzle out.
Also, he had wanted a large expensive toy, a Cozy Coupe, for the longest time. I explained to him that his diapers each week cost what a Cozy Coupe would cost. He agreed that if he peed in the toilet for a week, then I could buy a Cozy Coupe with the diaper money that we saved. Also, If he pooped in the toilet during the next week, I could afford the gas pump that went with the Cozy Coupe.
It took three days. He never had an accident after that. He loved his Cozy Coupe and gas pump for years. They were a great investment that saved hundreds of dollars.
I hope this helps. If you have questions, send me a private message or post in the Attention Deficit Disorder forum. I very seldom get to the Special Ed forum.