Perhaps just be honest and reiterate what you've written here. Disappearing for a few months at a time, doesn't quite qualify as carrying on a long distance relationship, never mind him just dumping on you, his plans to marry you.
And plus, there's mention of having a bf?
I often wonder how much pressure is placed on 'missed opportunities' , where 'love/the one' is concerned.
Granted, he may know things about you, that others don't, that doesn't necessarily translate to marriage, in my humble opinion.
Plus, the little red flag, that this thread mentioned in calling him crazy, a couple of times? Does he have a hot temper or some such??

Making distance a local reality involves many things. The hot and cold nature of this, doesn't bode well. Why now? Why not back when you'd met?