I don't think it's weird, but I didn't notice many (only a couple) of people at the pool with rash guard shirts (mostly children) when I was at the Poly, and only saw one adult with an actual t-shirt on at the pool. I don't find it odd, I don't really care one way or another, but I will say this I saw a man at the pool with a large scar down his chest and I didn't think anything about that either, I told my mom when I saw him "good for him, he obviously overcame something tremendous in his lifetime", and I of course didn't say anything to the man nor did I stare or anything.
I also have a large scar all the way from my neck to my elbow on my arm and it's very noticable to me (and probably others) and it cannot be covered especially in the summer, but no one has ever looked at me oddly or anything at least not that I noticed. I have had children ask me what happened, but to me that's not a big deal and I will tell them I was sick but that doctors made me all better (or as much as their parents will allow me to tell them).
If your son is self-conscience about it, I am sure a t-shirt will be absolutely fine, but really no one will probably give him a second glance either way, and if they do he should try and embrace his battle scars! I know that isn't easy, believe me, but hopefully that will be something he can come to terms with eventually.
No matter what I hope you all enjoy your trip!