Hmm. Been meaning to respond to this with a couple of stores involving myself and my sister. Might be a bit long, but it's the best way I might be able to provide some thoughts on the subject as well.
1981. Came back from DCA's to college and ran into a friend who marched with me at Westshore, also marched Crossmen and just finished playing Mello with BD that season. Went to his room for a beer.
"How long have you been at Westshore?"
"3 seasons."
He looked at me seriously and said,
"Now is the time for you to get out and move on."
"Why?"
"Different corps do things differently. Learn more than one way. There are a lot of great ideas out there, and the opportunity to learn them is important. Push yourself."
The conversation was great- he basically laid out all he'd learned at BD that summer. What he described to me was the basis for what's now "System Blue." I've never forgotten what he told me- to try and learn new things, expand my knowledge base, and keep in mind there's a lot more than one way to approach this activity that's valid.
Did I move on? No. Money was an issue, though Dad hinted recently had I wanted to go to Phantom, which I did not feel like I would fit in... maybe the money would have been there. Things were different back then regarding getting into a corps, less candidates... fewer cuts, etc. Another reason was that the management at Westshore laid things out at the beginning of that coming season, that things would be different, last season was disappointing, it would NOT happen again... major changes were being done, and we were going to take the next step in the corps' growth. There was no reason to doubt Larry Hershman. One thing about Larry I learned was that if you make a promise, you keep it, and he always did. A corollary to that was don't make too many promises so that you make sure you deliver on the ones you do make. DCI basically came to us that season in the form of some great new staff members, and I had a great experience over the next 2 seasons in that regard.
Next season, Ralph Hardimon came in for a rehearsal. Even the guys in the hornline were gassed about it. The next day over Sunday Dinner, I asked my sister how things went with Ralph....
"It was really good."
"Anything in particular?"
"He asked me to come out to Santa Clara and join the Cymbal line."
At this point, I did a spit-take and nearly fell over then blurted out
"You're going, aren't you!?"
Dad and Mom's eyes are bugging out at this point...
"No. All of my friends are here."
"All of your friends will still be here next year after you come back and tell us all about it! This is like Jesus asking you to be a disciple!"
There was come conversation with Dad, Mom was laughing.... then I said to my sister..
"I don't know about you, but if someone like Ralph asked ME to go there, I'd be packing my bags right now, heading for the front door, and telling you I'm getting to Santa Clara somehow!"
So... yeah- there is a chance to learn and expand one's self by moving on. Maybe the corps you're at WILL make changes to provide that- maybe not. Maybe you like where you're at and fit in, I'm uncertain whether anyone asked whether the OP's son feels like they really fit in and belong there anymore. Maybe if they do try out elsewhere, they find they don't like the scene. Maybe they do! I can't help but think the individual in question needs to "give it a go" as the Brits say and find out whether it's what they want, whether they have the right stuff, and like being there, or if not, go where home really is. I really wish the best of luck!