Don't give the idiot the satisfaction of knowing he got to you. If your
child goes to the meeting make sure he is in his comfiest sleeper or
onesie. Give the suit to whoever else has the next baby in the
congregation, he will then know you didn't appreciate the gift without
your saying anything. Sometimes the slow burn is best.
I like Lisa rose's idea the best so far but I also might return the suit to
the store and get several more Onesies for the baby to wear to the
meeting and if he asks about the suit, have your wife tell him to call
you about that.
It seems odd that a man would be giving the baby a gift to your wife rather than letting his own wife give it to her. This is what makes me think he's making a statement as an elder, about getting the little boy started out on the proper path and is making it seem as if he thinks of himself as your wife and your son's spiritual head.
You know the situation best but I must admit that part of me thinks that if it were me, I'd call the elder and ask him why he chose a suit as a baby gift. I'd let him off the hook and thank him if he said he saw the suit and thought it was cute and your boy was the only baby he could think of who he could get it for.
If he said he thought the suit would get the boy started out right as young minister or something to that effect, I'd know that this was another case of an elder assuming his authority goes beyond the platform of the Kingdom Hall. I'd remind him that as the boy's father, I will be taking care of the his guidance and will be deciding what is appropriate for him to wear. Depending on how the conversation went, I'd thank him for the suit and for thinking of your the baby but I also might tell him I would be asking my wife to return the suit to him at the next meeting.