I had a vasectomy about 8 years ago I can say it does change you a little. It didn't increase my sex drive but It did make me feel a little insecure about myself. If he feels the same way I did he may be compensating by wanting to have sex more to prove his "manhood". It should pass in time but he probably needs to feel like he isn't less of a man then he was.

I know he's not, so do you and he but the mind can make you feel strange feelings.