All my children went to state Grammar schools. It was damn hard to get in. However all of them reacted differently.
I must admit I never worried about them reaching their potential as I thought we provided them with enough stimulus at home.
I think all you can do is what we did.
There were tons of books in the house. TV was restricted. We all eat together at night and a lively discussion about Politics or Art, History, Science etc would always be going on, and their views were encouraged and respected.
We went on holiday and had as much fun as much as possible.
They are three Doctors, a Vet and a Pharmacist.
However, you know what I think it is more important for them ? It Is to be a happy person, content with life.
I would like to think that being happy is reaching their potential. It is worth a great deal to be content.
Two of mine are content. Two of them are ambitious and competitive and they are always onto the next project and never sit still. One of them is such an introvert, I don't really know what they are thinking.
From my own experience I think nature is a much more powerful force than Nurture.
I think you are a bit too involved in worrying about your sons happiness. You can't make his life totally free from boredom, it is impossible. Being bored is part of life.
If he is clever enough to get into a State Grammar and you provide him with a secure and stable environment, then you have done all you can. Let's hope he makes the most of what he is given. Many people are not so lucky.