I would continue to keep the pace slow as that really is the way to absorb history. Well, I would keep his assigned reading slow, and then supplement with other books or activities related to the same era and let him go as quickly as he likes with those extra materials. That way you are still spending a long time with a specific topic or time period, but satisify his need for more.
Or, he could go slowly with the history books you assign, but on his own, he could explore other historical periods at the pace he wants.
It really does make a difference when you go slowly…but that doesn’t mean he should be frustrated or bored. So, I’d look for compromises.
What a wonderful problem to have! 🙂