I found, with Eric, that it became more real as time went on. As early as you are in the pregnancy, he didn't really get it either... I got some books about having babies, and have been including him in a lot of the preparations as they come along - sharing ultrasound pictures, pointing out babies when we see them in public, getting him to help plan and shop and pick out things. Once baby was really moving and shaking, I got Eric to feel the baby move a few times, and having a big belly to point out to has really helped too! Involvement has really been key though, and I know it's especially hard with Daniel's ASD to factor in, but I'm sure you can find some ways that make sense to him. We do a lot of "what do you think" conversations too - what do you think baby will look like, what kinds of games do you think baby will like to play, what do you think baby will have in common with you, etc. We just sort of make it a regular topic for conversation without pushing too hard and let him come around to it.
Danny, of course, is going to be the problem. Eric's totally smitten with the whole baby idea. All of it is going right over Danny's head, and I'm a little scared of how he'll react once baby is here