There is an old saying that is geared towards mothers and sons, but I believe the same can go for dozens of relationships between a man and a woman.
"Your daughter is your daughter for life, your son is your son until he takes a wife."
I know you feel it is kind of harsh right now but it is normal for him to pull away as he is starting to get serious about a girl. This sounds like his first serious relationship and you can't really judge it because as you know with Kyle, relationships are all different. You can look at it like, he trusted you enough to tell you his deepest darkest thoughts/worries/fears what have you but obviously he wants to be with the girl and he is trying to make her happy, however the case may be. He may also feel a little silly for sharing what he has shared thus far.
I've noticed it is hard for women to let go, even for myself sometimes with my own brothers and I love my sister in laws to death, but usually you are not completely cut out of their lives. I know you miss all the good times but things can't stay the same forever especially as your lives take different turns. If the foundation is strong, it can withhold you both putting down roots
You have to accept the change and like you said move on.
I wouldn't make him feel guilty anymore if you really care about him and if I were you, if he were to come to you to try and vent about relationship problems, I would tell him you don't want to be involved because you will get your ideas about her and give your opinion and he will probably be mad at you when they reconcile so you'd rather not hear it. It's also not fair to hear one side to the story in my opinion...
Let him know you enjoy hanging with him when the air is clear so you can let loose, unfair tension only makes you uncomfortable. Literally, I know I tense up physically and point out the nearest exits mentally when people try to dump their problems on me and I can't get a word in edge wise.
I worry about you sometimes Lindsey! You are so nice, young and beautiful. You can't let so many things and relationships let you down and disappoint you girl.
You have a long life ahead of you, live life your own way and to hell with anyone who doesn't want to stick around for your ride. Remember, if someone doesn't make it to your future, there is a reason.
Never have any regrets. You got out what you needed, now let it be. That is obviously how they want things right now. Only time will tell.