Don’t look at letting him continue BMX as rewarding the stealing behavior. The two are not related. If you take away the one thing that he is good at, that he has a good experience with, that will very likely push him toward MORE inappropriate behavior, not less.
When parenting kids with ADHD, you should focus much more on the positive than the negative /weaknesses/ADHD symptoms.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism