If he needs it for medical reasons, why not -- as long as he isn't hoping for a happy ending. Send him to a 60-year old male sports masseur, not a 25-year old blonde female, though, and that should remove any question you have.
You might be better off asking your doctor than asking here.
The massage therapist would need a signed parental consent form and would take a detailed consultation to try to establish any contraindications to treatment. If there is any doubt about treating because of a pre-existing medical condition then signed consent from their General Practitioner would be required too.
I would have him evaluated by a sports medicine Dr first. at 13 and playing football, there could be other things going on besides just a few sore muscles
Make sure he stays dressed! The shirt can come off, but the pants or shorts stay on...though of course, a nice foot massage might help his back, so taking off his shoes and socks shouldn't be excluded.
I've actually given several back massages to friends and their younger brothers. They would take their shirts off, but 9 times out of 10, I would slip their shoes off as well and massage their stocking soles. If I thought or knew their bare feet might look good, I would tug their socks off too. I loved massaging their bare toes and soles.