"Son is a 2022 LHP. Painfree when pitching, decent about arm care exercises (I mean he is a teenager...) does what he's told to keep his arm healthy. He says his arm is "dead" after a long pitching outing. Not painful, just a little sore and can't generate much velocity/distance for MULTIPLE days, not just next day. Yesterday had to opt out of playing in the field for the HS team because he didn't feel comfortable with how far/hard he could throw (he's an outfielder when not pitching) after a 90+ outing earlier in the week."
So I would want to know a few things. Has this happened before and if so, how long did it take to resolve? How long ago has your season started? What throwing did he do to get ready for the outfield and pitching (talking about distance, number of throws, intensity of the throws both on and off the mound). Has that changed recently? Does he workout, if so, has anything changed? How does he feel overall (more tired, normal, great)? Any additional stress or sleep changes or anything else you or he can think of?
"Coaches say "ice it" but then seem frustrated when he's not ready to go two way. Pitching coach says to stretch and do bands and ice. Travel coach (who, like pitching coach, is a former MLB pitcher) says to "feed the beast" and get out there and long toss."
If you want the honest no BS answer, none of that will help speed up recovery. Eating well (you really don't want to be in a severe caloric deficit, for example, because you need fuel to recover), sleeping well (again, not doing so affects recovery), and managing load (this is more broad, but could include the number of throws, the intensity of the throws, stress both in life, school, relationships, family, weight room) are the big three. After those are in order, if you have the time/money to do other things (from stretching, ice, heat, massage, foam roll) you can. They do nothing to speed up recovery, but if they feel good and he enjoys any of them, go for it - just not at the expense of the top three listed (nutrition, sleep, adequate recovery/managing load).