Is he interested in learning to read?
My cousin was similar as a kid. Lego was an obsession. He was a very bright kid (one of the highest IQ's in Aust). The only thing he really did at that age was lego & learning to read & write etc. I remember he was much better at maths though. He was learning grade 7 maths at 7... So maybe learning to read & some basic maths? I only suggest that because his mind sounds like it works almost exactly the same as my cousin.
He was pulling toys apart about that age, because he wanted to know how they worked. His mum was going to give him her car to pull apart when she finished with it, just so he had something new to do!
He was an only child & home schooled. So his mother had all the time in the world to put all her energy into him & his interests exclusively.