ADHD advice please

'lo everyone, how are you all? Its been a while since I've been around - up to my ears in coursework and minding........

Right, need some advice please, I mind a 10 year old who has ADHD. He doesn't particuarly want to be "minded" but I have his younger brother and obviously mom pays!!! I have them varying hours each week - some weeks it earlys before school other weeks its after school for 2.5 hours, holdiays various full days.
When child is in a good mood he is brilliant, co-operative, sunny, helpful, communicative etc, he is great with the 2yr olds and under. Obviously he can be a bit "high" at times but I have noticed he is particuarly argumentative and combative with one of my other mindees a 9 year old who is quite quiet. He can be like this at times with my own children too, age 9 and 6. My mindee 9 year old has told his mum that he feels uncomfortable when 10 year is present and I have noticed that him and my eldest will go of on their own (ie to sons room). I have tried to implement a "behaviour" agreement between me and all the school age but the 10 year olds view is (please excuse me) "thats gay and I'm not doing that", I have made a little pack for my school ages with this agreement, explanations of expectations, rewards, good behaviour and a questionnaire all about them. Apparently the rewards a "naff and not enough" - I have stickers, certificates, sweetie tin and a "dip box" full of little goodies (ie whoopee cushion, squishy thingies etc). All the other kids think this is a great idea and even my own sons have agreed to sign the "agreement" forms.
I am trying to find out about ADHD as much as possible and I am trying to pin mum down for a meeting to discuss this.
Anyone with any experience of this would be helping me loads and all suggestions received with THANKS.
How much of this is/could be ADHD, what can I expect of him - even with ADHD does he have some control over what he says/does at the age of 10, understand right from wrong etc? His behaviour for the first few weeks with me was spot on - although he got a little too playfully aggresive at times. Bear in mind that I have these kids from 8 - 9 am so surely his medication should have kicked in round this time? I know they all want to let of steam AFTER school etc.
He told me this morning that I can't tell him what to do and that he could just leave if he wanted to.
I always keep calm (even though I get very frustrated), talk to him calmly and rationally giving him chance to voice his opinion etc but I cannot have him hurting my kids, totally ignoring what I am saying, making other mindees uncomfortable and potentially walking of.

HELLLLLLPPPPPPP Thanks