I don't think MAP would require consent. As long as they give it to everyone, and they aren't singling out one kid, I don't think that consent is required. If he isn't doing academics at school, then even if the school uses MAP scores for placement, it won't matter for your son. I wouldn't worry about it.

They do use MAP for some placements in our district - my 4th grader just took the MAP Math for 5th grade placement. The most advanced placement is for kids with 96th+ percentile scores.