I am so sorry for the loss of your relationship, I can't comment on his current relationship.

It does sound though that he wasn't sure that things were right for him .
It sounds like he had ALOT of doubts but rather than just tell you he tried very hard to say things he thought would hurt you less.
Instead of which he has left you more hurt and confused.

He sounds young and simply not ready to commit. Which he did say in a round about kind of way, when he said he didn't want to marry the first person he fell for.

I am guessing your both quite young, since it's seems to have been quite an intense relationship over a short amount of time.
This in no way lessens your pain or depth of feelings on the sincerity you felt about your relationship. It does help explain his perspective though.

Your only way to move on is to stop checking on in him and don't ask his friends and flatmates about things, it's not fair to put them in that situation.

It may well be that this is a rebound relationship, but that doesn't change anything for you. Nor does it guarantee he will come back.
I have had my fair share of rebound flings, they served a purpose, then I moved on.
Sometimes we just have to accept we won't get all the answers.

I wish you all the best and hope you are able to heal.