What to say when a guy bails

So I started sleeping with this guy a few months back. And he's lovely and seems really interested.
But he has a tendency to not respond and has bailed A LOT.
He has uni and exams so I know he's busy. But it's happened too many times now and I feel like a pushover.
I don't know if he speaks to other girls or maybe that's just my mind wandering. But I can't can't shake the feeling that something isn't right.
He has just bailed again and said 'Can't tonight sorry, I've got so much to do, sorry babe xx' 
What do I say to him?
I don't want to sound like a winging bitch but also don't want to be a pushover and let it continue to happen. Because I do like him and it's very annoying