Although very few people talk about it, I don't think this is an uncommon reaction to birth for many men. Not all guys are up for being a "birth coach" or really want to see their wife/lover in quite such a... primal way. It's one reason men were never traditionally allowed in the birthing rooms (this all changed in the 1960s and 70s).
So, unlike the other posters here, I really don't think you should make him go. Personally, I offered my husband the option of being my "primary coach" or not when I had my son. I did this out of consideration for his feelings and after having talked to my brother in-law (his brother) who had been at the birth of his daughter. My brother in-law had commented that he was excited to see his daughter born, but at the same time would have rather not been there in retrospect - it was all just a little too "real" for him. Plus, I think some men just cant get the birth scene out of their heads later on when it comes time to be intimate with their wives. I think it's wussy of them, but I can't really blame them for it.
I suggest you get a doula to support and attend you with this next birth. Your husband is clearly more comfortable being a dad to your son, and I think you should let him feel useful and helpful by doing that. A doula, on the other hand, can be there for you in a capacity that it sounds like your husband wasn't the first time around. It's her job to help you through the process, support you, make suggestions when you need it, etc. Plus, lots of well-run studies have now shown that having a doula present can shorten your time in labor and lower your risk of interventions and a c-section.
Good luck with your birth!