I do not know the answer to the reasoning but your son's doctor should be able to explain why they want to go this course of treatment first.
One guess is that they think it might be an easier compliance issue for someone his age, they do not have to go get infusions or injections - they just take a pill once or twice a day. I'd rather have an infusion once a month or every other month. I have a car and can make my own schedule -- easy for me.
I definitely think logistics was a factor in the doctor recommending the Zeposia. Since my son is going off to college in the fall, it's easier for him to take a pill than figure out how/when to get an injection/infusion. That being said, those things can be worked out. I don't think they should be the main reason to recommend one drug over another.