This is so school-dependent that we can't really say. But tbh I'd worry more about Alg1 being an anticlimax than about it being too hard! It's possible the school will pile on busywork homework, but as the school is being helpful now I would cross that bridge when I came to it, personally (eg by having him do only the harder problems).

Fwiw, without knowing your son, the plan to some work over the summer and go into Alg1 sounds fine to me, if he's happy with it.
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