I think a 2 yr old can get it on the most basic level. Just keep telling him a baby is in there. If you're comfortable with it, let him watch some birth shows (of humans) where moms give birth. It'll make it make more sense to him. And, he won't be traumatized...at least mine weren't. hehe.
Even if he doesn't get it, odds are he'll be just fine. My son was 19 months old when his little sister was born. I have no idea how much he understood about the pregnancy, but he did great with the baby being here. Also, when that daughter was 22 months old and her little sister was born - same thing. She did great with the new baby (I keep my little ones as my "buddies" and they help me with everything. I do this so that they dont' feel left out.). But I'm not sure how much she understood about the pregnancy itself.
I have let them watch birthing moms on youtube to get a better idea. They find it fascinating.
And, as for your hubby...very cute. Someone else made a really good point, while the baby/you are fine with him climbing on you, when you get bigger, it's not nearly as easy. I'm 35 weeks right now, and my 14 month old and 3 yr old and 4 yr old like to climb on me as though my belly doesn't exist. I mean, the older two enjoy that there is a baby in there, but I am having the hardest time getting them to be more careful about squishing me and such! It definitely gets more uncomfy as you go along...at least it does with me.
That's funny about your hubby, though. I don't know if you can get him to relax. Guys have their own emotional thing they go through with pregnancy:-) hehe.