If you have confidence that he knows the prealgebra, and it sounds like you do, then there is no point sitting through a summer class.

Are there any implications for him in taking the Algebra 1 class? Does it cover everything and therefore potentially make sitting through it in year 7 tedious? Or is it a taster course which will kickstart his progress in yr 7?

If it is a comprehensive course can he then test out of Algebra 1 at school and do something else?