Not quite the same, but we just got home from Ethiopia with our now 22-month-old son in July, then got pregnant 2 months later. Oops!
As an adoptive momma who was just recently in the waiting process, I can totally relate to not wanting to put the adoption on hold. However, with a new son home, I can see why it would be difficult to bring a child home and then give birth only 4 or 5 months later. In terms of attachment and transition, 4 or 5 months just really isn't very long to let the adopted baby be the baby. You might want to ask your agency's opinion on this—most strongly recommend one year between a placement and adding another child to your family, either through birth or adoption. Attaching in adoption takes hard work—I can't imagine being 8 months pregnant right now and having to divert my attention from my new son so soon after he came home. I really feel that it would damage his attachment and feeling of security in the family.
I know this is hard to hear, but I would give some serious consideration to your husband's suggestion. Talk to others who have been in similar situations (PM me if you want me to put you in touch with a few people), maybe set up an appointment with an attachment therapist to get his/her opinion, and talk to your agency. I really and truly understand how devastating it would be to have to delay the adoption at this point in the process. However, I think if you can put those thoughts aside and really spend some time thinking about what's best for your adoptive child, the son you already have at home, and your newborn, you'll get the right perspective to make the right decision.
Good luck with your decision, and please feel free to PM me if you'd like to talk it through more.