Sudden Lack of Affection in a Partner

I've been dating this guy for about 6 months now. Up until around two days ago, this guy has always been pretty affectionate and very good at responding to texts and checking up on me. However, I've noticed a sudden shift in behavior from him. Usually whenever I tell him I love him, he'll say it back or he'll text first me saying he's thinking of me, etc. But now he's stopped expressing any affection and doesn't say "I love you" back to me. He's also been taking a few hours to respond to me, which is out of the norm. I asked him today if there was anything bothering him because I noticed he was acting differently (I didn't specify further than that), and he said he's just been feeling sad and nostalgic lately, and he gets sad when he goes back home and realizes how simpler things were when he wasn't in college. Of course, I understand that this sadness could be causing him to pull back, but I feel like I needed to get this off my chest and get input from others. Could he actually be losing interest in me, and maybe he wanted to wait until we were apart to try and push me away? Sorry if I sound paranoid about this, but this is my first serious relationship and I don't know where to turn to

Sometimes partners have different ideas about what the relationship means to them. Have you ever asked him what or how he regards the relationship between the two of you? Have you considered what or how you regard this relationship?

The answer(s) may or may not be what you want to hear, but if you want honesty in this relationship, it may help clarify what you can expect in the relationship.

People cope with stress differently.
Some will vent until they have nothing left to say, and others will shut down and get quiet.

If this is a temporary thing, there is no reason to make a mountain out of a molehill, but don't let HIS stress become YOUR stress; you can be concerned for him, but you can't sacrifice your happiness and sanity. If it bothers you to the point where you start to feel paranoid or insecure then I'd just ask/confront him directly.

Tell him that you understand he is going through a lot but him not being as affectionate or attentive makes you feel left out and unloved. And that you'd appreciate it if he can talk to you about how he feels so you can understand and support what he is going through. That you two are in this together and you'd love to help. Perhaps he is waiting for you to initiate this; sometimes I'd expect my partner to notice that I am sad or distant and pull me back in without me coming off as "whiny," even though it's extremely illogical to expect someone to read your mind.

But if you never communicate and fake your true feelings because you don't want to seem vulnerable, nothing will change. After you are honest with how you feel, he can either let you in or reveal to you that he has other intentions.