I agree that disrupting school might be an issue. Definitely get on the waiting lists at a couple different facilities now. If there’s one time a week when your son wouldn’t be missing much in school (a lot of the early elementary years have more free time in the schedule), then I’d try to schedule for that time. Once you take a child with ADHD out of school, it’s really hard to get them to go back and return to focusing on being there, so you might find that it just isn’t worth the disruption.
We have 1-2 occupational therapy offices that offer Saturday hours. Those may be full where you are too. Really, the best thing to do is get on several wait lists and take what you can when it comes available, unfortunately.
There are many OT activities you can do at home in the meantime. This article mentions a few:
ADHD coaching is usually done with teens and adults, although you can find parent coaching for raising kids with ADHD (and parent training). I haven’t heard the phrase “ADHD rehab” before. Maybe ask this clinician specifically what types of therapy and professional services he or she is recommending.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism