Definitely NOT true to size. My son is 115lbs and 5'1-5'2". The small was too small and very tight. I don't think medium is much better. I ordered a large. Hope it fits. Fingers crossed there he needs one this weekend.
Pros
I bought the men's XXXS for my 60lb 8 year old and they fit great!
Cons
The seams at the neck came undone the first time I wore it.