I am still dragging DH, kicking and screaming, into the light and the only things that have really made a difference are DS's psychologist and DH's own eyes. Having a third party, who also happens to be a 'real' expert, share observations and concerns, helped enormously. I get the most buy-in when I am able to point to concrete examples of the psychologist's concerns in action.
I also use the same approach I sometimes use with the school: talk as though of course he knows and understands what I'm saying and is in complete agreement and I'm just engaging in idle chit chat by saying it. That lets him go away to ponder on his own time.
There is a fair bit of worry and wishful denial in DH's reluctance to explore the issue. If that's a factor for your spouse, it may be best to give him some time to get used to the idea, punctuated with the occasional comment about a positive aspect of giftedness. The potential problems can get overwhelming.
