He thinks he's too old - then yes, for him, his current age is too old. Just as, for me, having DC at 19 would have been far too young, but for SIL it was fine.
It's a personal decision, and he's decided he can't deal with any more DC. "He can now effectively decide 'thats it- no more'" - well yes, he can, you can't force someone to have a baby.
It's a discussion that comes up time and time again, and it is a terribly hard situation to be in. However, when it comes down to it, I think the worse thing would be to have a child when one parent doesn't want it; bringing another person into the world when one parent explicitly doesn't want it is a dreadful thing to deliberately do to a child.