Sorry to hear TX! As stated, Rick can help with the rules. If he was not on scholarship and was not recruited, I believe that there are no transfer restrictions, but again, ask Rick.
Lots of good advice above. He needs to reach out to as many programs as he can to put the word out there. Getting in the portal as well. If he was good enough to get into a P5 as a walk on, he should be able to find a home somewhere.
Keep his head up, speak kindly of the former program, grateful for the opportunity, but it just didn't work out, learned so much and ready to work hard with new program. He needs to stay positive, even if it wasn't. New coaches don't want to hear the negatives. They want a player ready to move on and add to their program.
Most here will tell you things happen for a reason, and mostly work out for the better. Best of luck to your son and keep us in the loop as his journey continues.