This has probably already been said, but yes if he was recently neutered, he will still mount and they can still tie up. I found this out many years ago when we neutered our male. He was about 10 months old and already aware of his "ablilities". We had him fixed and our female went into heat with in days of his surgery. They tied up - because the swelling has nothing to do with the actual possibility of ejaculation.
I called my vet in a panic and he told me that they can actually continue to breed up to 2 weeks after surgery. I was like, Oh great! But he told me not to worry. The surgery was done completely and the dog should "get over it" in a few months once the testosterone dies off.
Just an FYI, our boy is now 11 and he'll still go after anything in heat! It's like he just doesn't know he can't really do anything. His drive is really, and amazingly, high!