He is older than your boy, 19. In college. 6' 2" 165 lbs. A bean pole still.
Although he has gotten much stronger in the last year.
He is mid 80s.
You said your son was throwing, it sounds like throwing pretty seriously, 3 days in a row. The reason I brought up what my kid does is does throw 6 days a week, but, on those "in between" long toss days he is just playing a light catch mostly. Arm care is really the main thing in his program and is done every day. I consider long toss basically part of arm care.
Long tossing, as I am sure you know, can have an ebb and flow to it, just like any exercise. Max distance one day and not the next time, that is normal. It can vary.
As to what Roger said, he is correct. If his arm was just tired that day taking a quick break while doing other things should be fine. If he really has "dead arm" and has overtrained or just way over extended himself with all the gym work, throwing ect., that is a more serious thing and can take some time to get past.
He is also correct in that proper sleep and nutrition and proper hydration are often lacking in young guys that are working out a lot. They want to do everything, hang out with friends, train, play games or multiple sports at the same time. For every thing that is done there is a recovery that is needed.