You've said he's OK with not going on holiday. He's not. If he was he would have just cancelled it.
This. This is what you need to listen to.
He is making you make the decision. He's telling you he isn't bothered, so he is coming across as all so reasonable, hoping you'll not want to look like a jealous GF and say he can't go. I mean he's being so reasonable about it, right?
He has even got YOU feeling bad now, for not wanting him to go! He shouldn't even be putting you in that position. HE should of immediately cancelled the damn holiday ffs, It shouldn't even be part of a conversation right now!
He cheated on you, and if he was really wanting to work on your relationship and regain your trust, going to ibiza on a bloody lads holiday should be the absolute last thing he would consider. And yet, he hasn't exactly NOT considered it has he? He hasn't cancelled it yet because he's laying that whole decision at your feet!
Instead, its all on you now, whether he goes. Thats lovely of him isn't it!
Of course you can't tell someone they can't go. I think I'd go with 'do whatever you want to do'. and then when he jets off, get rid of him.
Someone who loves you does not cheat on you OP. Ever. At all. And they certainly do not then even consider going on a bloody lads holiday afterwards.