This is what I was going to recommend. Take your existing reservation and break it up however you need to so Disney knows there are 2 nights where 3 people will be in the room.
If your husband is there in the middle of the trip, nights 4 and 5, you let them know ahead of time. They'll break the trip down into three different reservations, nights 1-3, nights 4-5, nights 6-8. They'll warn you (because they have to) that as a result of the split they can't guarantee you'll remain in the same room, but I've never heard of anyone who split up their reservation for any reason ever needing to move. It's a CYA thing in case they screw something up, that's all. But if you do it this way, they'll charge you the extra money for the nights he's there, but only those nights and everything's kosher.
The other option is the Slightly-More-Honest-But-Not-Totally approach of alerting the front desk on the day he arrives as if it were not planned (and hey, there's always the possibility he won't be able to join you after all, right?). At that point, the front desk can essentially choose to waive the fee (which might happen though I wouldn't count on it), or they'll do what I described above, cancel your existing reservation, set up a new reservation for those 2 nights with 3 adults, plus another reservation for the duration of the trip back down to 2 adults. The only drag with that approach is the time you'll have to spend at the front desk making that happen instead of enjoying yourself.