First, I'd say, calm down. Try (operative word) not to be sensitive. Some guys think this adult "teasing", "jabbing", banter is good natured fun. They don't think they are being rude, they think it's just "guy talk".
Second, why not try to "join" in the banter instead of taking offense. There are ways to "get back" at his comments by making some observations of your own. Perhaps, "Well John we're not raising a herd of elephants here are we?"
Third, who cares if your son is a little clingy - he's 15 mos old! Some kids are skinny, some are average and some are fat. It's not RUDE, it's just a fact. So what if your kid is a little skiddish around him or other family? The reality is he IS still a LITTLE BOY. So yes, he's not going to act like the big boys. But I will also say MANY MANY of us change/have adjusted our parenting style between child 1 and 3.
Yes, many of us "babied" our first one. Because they were small, adorable and we loved them to pieces. Just know that for many of us, once the 2nd, 3rd, 4th, etc. comes along you just don't have the time or energy to "baby" your babies as much. They need to walk, talk and stick up for themselves!
What I'm saying is, sometimes a fact is just a fact, even if it seems like he's being rude. He may be pointing out the obvious, just in that rude TONE that you don't like.
I understand this is really bothering you. So I think you either need to give him a taste of his own medicine or ignore him. I don't think you're going to change him no matter what you say or do.
And yes, I think you are being a little sensitive about your little boy. Someone will ALWAYS have a comment you don't like. If you are a quiet, mellow person, of course your BIL and his kids are going to seem like a herd of elephants! Just know that it takes "all kinds" to make this world go 'round...someday kids like your BILs kids are going to be your kids' classmates, on his soccer team, etc. So get used to it now.