Thank you for your compassionate and compelling question.
It could be that he no longer wants to be married. But, that should not be construed to mean that he does not care about you or love you. It could be something to do with him.
In actuality, he may be depressed, and may also be Gametaphobic. http://www.panphobia.com/events/gametophobia.htm
However, I see signs of depression, in what you say, that lead me to believe it is more related to that than any phobia he may have.
1. he is asking for space, but
2. has become a hermit
3. sitting at home by himself
4. isolating himself from family and friends
5. isolating himself from those who are close to him, not just you.
6. missing school (a low level of dropping out or isolation again)
7. Oversleeping, (not hearing his alarm)
You sound like a very caring and wonderful person who is taking such an interest in her partner, that you even notice these things.
I would like you to not internalize this as being about you. But would rather like to see you take a helping and listening approach. If he is suffering from depression, which I think he is, then when you introduce questioning his trust and or love, then it adds to his burden. For example:
Maybe he is failing at school, failing at work, and now his relationship is failing.
Of course in the end, it may well be that he is not into you, and perhaps he does not know how to break it off.
let me suggest this.
1. First of all, if he is suffering from depression, it is not your fault, and is not your responsibility, per se. however, as a loving partner, I sense that you would want to help, if you can.
2. Take the pressure off with regard to the relationship and trust. Replace it with asking him what is wrong. Let him know: look bob, I see you are not happy right now, do you want to talk about it? How is school? How is work? Is everything ok? and so forth
3. If the issue is you and him together, it will eventually come out.
The main thing is for now, to not force the issue of trust and relationship. He may not be ready right now for that.
I really do not think it is a trust issue based on what you have told me, unless it is a larger issue of trust related to women or people in general. The real issue may be depression or some other issue going on in his life.
Note: one thing to not discount. I do not know your fiancee or his character. So as a third party looking in, I need to make you aware, that besides depression or other emotional or psychological issues, drugs can also produce this behavior.