He's a 2025 and a RHP? Training seems like the no brainer to me. I say that for a few reasons. First, my 2021 son has played different versions of fall ball off and on since about age 7. Yes, they're a way to get extra reps in, but the quality has consistently been crap. On its best day, its a step down from spring and summer ball. Football is big in our area and it takes a LOT of the baseball talent out of the mix in the fall. To make matters worse, our experience is that coaches, players and parents are way more laid back about it in general. It all adds up to much less juice from the squeeze.
While son starting lifting around the same age as your son, I didn't see it produce any meaningful gains as a freshman or soph in high school. As far as I can tell, he simply wasn't lifting seriously enough. It was 3-4 days a week and felt closer to the bare minimum. He simply didn't love it enough at that time. But his last 2 years of high school, he got 5-7 days per week serious, stayed in the weight room longer each day, and started getting protein and creatine into the mix. Once all that happened, the results became easily visible. Not just in his build, but in how hard he was hitting and throwing the ball.
Now, is your 2025 ready to get serious about training, lifting, etc? No clue. But if he is, he can do a LOT with the time this fall. A helluva lot more than he can by playing fall ball as a rising freshman. While some will disagree, I lean towards believing that the human arm only has so many throws in it. Assuming that's the case, I wouldn't waste any of them against fall ball competition this fall. It's a marathon; not a sprint.