The apple doesn't usually fall far from the tree, so I'd start out by asking him what his early school years were like, and see where that leads him. In my case, I was already aware of how badly things went for me in those years, and when I noted DD hitting academic milestones ahead of where I did, I was already in panic mode.
DW's experience was quite different, largely because she was so busy catching up in language and shifting from school to school that she never had time to notice how advanced in other ways she was. She saw our DD and thought "normal," because the only other toddlers she'd spent significant time with were her own family, who she hadn't yet learned to recognize were atypical themselves. In her case, she needed to see what "normal" actually looked like. Taking DD to dance classes and library readings were an eye-opener. Another eye-opener was looking at the graduation requirements for kindergarten, much of which our DD had completed when she was 3. By the time DD went to pre-K at 4, and her teacher declared, "I don't have anything to teach her!", DW was fully converted.
That was very convenient timing, incidentally, because it never became a problem until DD was enrolled in K.
