You are correct that if he is rejected ED he cannot apply RD for the same year.
Chances are higher for ED but Stern is a reach for everyone (overall admit rate 7-8%, haven’t seen a split in ED vs RD rates). No one can be totally confident with admit rates like that. If Stern is his number one choice however he should apply ED, it will be his best chance. Put it this way, his chances at CAS RD are unlikely to be better than Stern ED.
UCLA may be a reach too (you need to look at UC capped weighted gpa), as well as Amherst. I presume he has some matches and a couple of safeties on his list? Vassar is the only school on that list with an acceptance rate over 20%.