Why do you want to marry someone who's cheated on you?
Of course marriages can go wrong, and people sometimes cheat on each other. But if he's done that BEFORE you've got married he's already shown you he can't be trusted and he doesn't value you enough to be faithful to you.
It sounds like he gave you a fake prooposal to wind you back in so you'd take him back, thinking he'd 'changed'. If it was a genuine proposal he'd be keen to set a wedding date, and not 3 years ahead!
He is showing you that he doesn't want commitment. You know he's capable of cheating and lying to you. You could get married next year without it costing lots. EVEN IF he genuinely is keen to have a big fairytale wedding (does that sound like him??!) does he honestly think that his requirement for a big 'do' is more important than your mum being there, and well??? Pretty selfish of him, even if that is true.
But of course it isn't. He's even said he 'isn't ready'. So is that the real reason? Or is it the cost? He can't even get his excuse straight!
Please don't marry this man. Please leave him. He will mess with your head, stringing you along, just saying enough to keep you hanging on. Then one day you'll realise another 5 years have passed and you still aren't married and don't have commitment from him.
You deserve better. You deserve someone who loves you enough to realise your mum being at your wedding is important, someone who doesn't string you along, and lie and cheat.