I have no idea what he means. Unless your boyfriend is having an existential crisis, questioning the very nature of life, society, and... death.
He has come to understand that, truly, the cradle rocks on the precipice of the grave. Death's cold hand rests upon all of our shoulders, at all times. From the moment we can walk, we are slowly truding towards oblivion.
And what if there is no afterlife? No heaven? What if, instead of cosmic judgement, there is merely... nothing. If we're all just meant to be worm food, then what is the purpose of existence?
Is life meaningless? Why not cheat? Why not bang whoever we want? WHAT DOES IT MATTER!
Man, your boyfriend is deep. Either that, or he's a self-absorbed Momma's boy with entitlement and impulse control issues. Probably that. You shouldn't forgive him. Not because he cheated, but because he tried to sell you a line a stone goth kid couldn't deliver without snickering.
I could be wrong, of course. "What if there's nothing after this" could mean... actually I have no f*!#king idea what he's talking about. Ask him to further define this statement, then get back to me.
The more I think about, the less inclined to think this was a statement addressing mortality and the reasons human beings need to behave according to basic rules of behavior regardless of what comes next, be it a void or a cloud full of bird people in togas.