"Gently surprise him?"
I'm thinking that he will be concerned about how you got pregnant when he's shooting blanks.
I think if it were me, I'd call his doctor that did the procedure and ask him how this makes any sense. Perhaps call your gynecologist too. Certainly, get a professional test from your gynecologist. Get something to help you explain how he's shooting blanks, you are breastfeeding, and you haven't had much sex, yet you are pregnant.
I think your husband will appreciate that you were concerned about why it didn't work and find out if these procedures are only 95% guaranteed or what. Maybe you or he has some unusual condition that allowed this to happen. I sense that you may be happy about this so tread lightly in case he will, naturally I think, have some lesser positive feelings about whether or not you were faithful.
Also, its just me, but I would never assume that you know your partner's deepest feelings on a subject like this. They can change and we don't always share our changed thoughts with our spouses as there is thought to be no need to.