It depends on the state where he wants to teach. If he’s sure he wants a music Ed degree and plans to teach, I’d recommend just doing the bachelor’s degree. A music Education degree has a ton of requirements (many do overlap with performance, it just adds a bunch of extra) and it’s not something to do if you’re not sure. He can find a degree plan as a google doc in silent advisor and compare what he would be taking as a performance major vs music Ed, and ask the advisor how things are sequenced in the dual degree program and whether there’s room for any electives at all. Does he have any AP of dual enrollment credit? That can free up from for outside coursework for a music student.
There are plenty of post baccalaureate teacher certification programs. It does vary state by state, and the next question is, if he gets certified in a different state than the one where he plans to teach, is reciprocity offered for the teaching credential he receives, to the state where he plans to teach. Lots of variables.