I'm a music teacher (in the middle of the Pacific so can't help directly sorry
) but here's my thought for what it's worth.
Call the school and explain that you will be looking to barter for lessons and a keyboard. Just be matter-of-fact about it. If they are the ones who have someone skilled to teach your DS, go with them.
I barter with students for childcare, personal organization stuff, etc. If a student needed access to a piano, I'd give them the combo to the practice rooms at the local univ. where I also teach.
This is not an impossible situation, logistics-wise.
I also get that you had it settled and are dealing with a new revelation about DS's father's behavior and attitudes. It's really beyond the pale what he did.
But when it comes to getting lessons and an instrument, there are teachers and instruments littered all over the place. We get into this because we have lotsa heart. We usually need help and will barter. We always have ideas for how/where to practice.
Seriously. You can do this. Hugs.