This is a tough one! I understand Mom's desire to try massage first, because I prefer natural remedies whenever possible, too - but doctors are good for something, and the initial assessment by a doctor could rule out a broken rib, punctured lung, etc. that could make massage dangerous to the little guy.
I think I'd ask her to see what the doctor says, first. That is walking on thin ice with some people, but six is very young and tender,
If the doctor doesn't find anything of interest, he'll say just to give it time, rest, or even Children's Tylenol. If that is the case, I'd gladly give the massage, assessing the muscles carefully to see if you can find the problem. It may be hypertonicity or a trigger point. Probably, since Mom's instinct is usually right.
Western medicine is great at diagnostics. Eastern medicine is great at promoting true recovery without the side effects. That diagnosis is incredibly important pretty often, as much as we wish it wasn't so.