You should have turned towards him and said "that's my baby over there, and he's doing fine thank you"

I feel your pain, OP. my coworker had the same name as me and gave birth to twins. While she was out, people would ask me how the twins were doing. One lady would not accept the fact that it was not me, and continued to loudly insist I had twins. My future husband was working 10 feet away.

Then you should have replied "that's not me - but i will tell [colleague] you send best wishes for her baby" ... or something that clears up the situation in a friendly way

I can't tell you how often I have this problem at my job too. At the first store that I worked at, I was confused with this co-worker of mine because we both had dark brown hair, wear glasses, and are heavy set. The difference, is that I have freckles and she had acne, I'm taller, and she's more rounder than I am with slender legs where as I'm thick all over. Last year I got transferred to a different store, and then I got transferred again in October. Both stores I had people say that I remind them of this Megan chick who apparently worked at both stores that I've been transferred to... My hair is red now, and according to everyone who knew her, her hair was black. WTF? How is that a resemblance??? lol

I mean you could just explain. Why are people incapable of this?

You are 25 and you have a "crush"? Get a life.