It sounds like it would be too early to know if OP’s S will be interested in grad school. Whether the quarter or trimester system could work might depend on whether he could stack his classes to the off-season or not, and how much the teachers work with him. I once took some classes at Villanova, and they videotaped each class and you could access any recording from your computer at any time with your class password. I don’t know if they still do that, or if anyone does, but that’s a pretty ideal situation for kids that have to miss class for one reason or another.