I would think only getting B's would be a reason in slowing him down.
Agreed-- I would look at what those grades are based upon, and what is keeping them B's and not A's.
In most middle school settings, that would (to me) indicate that mastery isn't quite there with the material. That is, understanding is "solid" but it might not be truly mastery.
Looking at the scope and sequence-- and texts, if you can-- for all three courses would be my suggestion. Look at what is covered by the course the student is taking now, what would be covered by pre-algebra, and what is expected of students in Algebra I.
It might be fine in light of the student's history, but there are going to be a lot of small contributing things to consider.
