I agree with everything so far. You can't sacrifice your education to spend every minute with him. The fact he is over here for uni and you see each other 3 or 4 days a week already puts him WAY ahead of the game as far as people from two different countries in a relationship getting to see each other. If you were both in the States, going to uni in different parts of the country you'd be lucky to be able to see each other 3 or 4 times a year, not counting school breaks.
He has to see that there is a huge difference between you needing to go to class, work and have study time... and his friends who don't work being able to go out more.
I'm also thinking longer term, bigger picture, and you can tell me to butt out if you want. I am wondering if his pressuring you to spend more time with him and using the guilt of him making the sacrifice to come here is a sign of how it will always be. I don't mean to be Debbie Downer, but it is just a bit of a concern that he can see the time needed for work, school and studying, plus travel time and costs, and is not satisfied with you making the effort 3 - 4 times a week. Does he make an effort to see you? I know public transportation can be costly too, but if you come to him he should make the effort to come to you. Like I said, tell me to butt out if you want.