Ask for specifics of what you need. It sounds like you don't have a lot of time together with him being in school and you working long hours. Relationships do need time and attention to keep functioning. It sounds to me like you need some dedicated time to really be with each other without other distractions. I'm not sure how much time you can carve out to spend with each other but find that time and really commit to spending that time together.
He's right that one can never know what the future will bring with how you will feel about another person. Can you try reframing that statement in your mind though? When I really thought about it I found it comforting that my partners are with me everyday because in that day they chose to be, not because of some outside force.
Everything will be ok in the end. If it's not ok it's not the end.