If the trouble is ADHD, keep in mind it is not a "behavior" issue, but a neurological one. So, not a willful act of not paying attention, showing work, sustaining a work ethic, but a neurological difference that hampers his ability to do these things. Goals and rewards can help - but as eema suggests, they need to be fairly specific and shorter term. You might want to try a reward for showing his work on 5 consecutive days of math homework (and reminders before he starts each day) rather than a reward for an A in math. It's important that the reward be fairly immediate.
annalissa raises a good point about the transition from Montessori to a traditional educational model. Be sure that he fully understands the routines, expectations and has the skills to carry out the expectations.
I do think that kids that struggle will adopt an "I don't care" persona to cover up for their distress. I truly believe that almost every child wants to be successful - and when they start to detach themselves, there is a reason.
