If your dh is gifted, and he probably is, he might think that he was smart and managed OK at school.
I know for my Dh, one particular book was eye opening, Gifted Grownups by Marylou Strenewszki. He did not read it himself, but as I was reading it, I read aloud certain passages to him, or passed the book so that he'd read a few pages. We both went through an interesting re-examining our lives period from that. He was totally on board after that.
I have recommended the book to others, and have heard good things. A father who was resistant about having his Dc in a special gifted school, and completely changed his perspective.
