I believe the proper response is to send a brief note along these lines:

"Congratulations on the upcoming marriage of your daughter [if RSVPing to
bride's parents]. We are deeply honored to be invited. I wish I could
come, but am not able to leave my five-month-old son with a sitter during
the reception because he is exclusively breastfed per our pediatrician's
recommendation and will not take bottles. The celebrants are very important
to me, and please let them know that I would not miss their wedding for the
world except for this. Please accept my deep regrets. KD"

If you are particularly close to one of the celebrants, it is OK to tell
them by phone why you are not coming, but it is important to state it as a
sad response and not as a request for an exception. The request is implied
in giving the reason; but stating it as a response allows the parties to
save face if they do not want to make an exception. Wedding plans can be
stressful, and you never know what force of nature in the couple's lives is
behind the no-babies rule. It's best to just go along with it. It's
supposed to be the couple's day, and so rarely is, so an invitee's needs are
best left low on the radar.

If they are able, they will respond for you to by all means bring your son.

For the kind of money you're talking about, you could also consider flying
them out to see you for a long weekend a month or two after the honeymoon as
a special treat.

HTH,
Dagny