I think deep down this really isn't about revenge or sticking it to him. You say you have evidence he has booked the trip (and therefore paid for it) so you could confront him now. There isn't really anymore evidence in April than there is now.
It's about a fear of coping on your own, emotionally and financially. You are too scared to leave so you continue to shift the goal posts further and further back, hoping he will redeem himself or prove you wrong so you don't have to leave.
Having your son there to protect you while confronting him is a really bad idea and is not fair on him. If you won't leave before his holiday, do it while he is gone. Who cares about revenge, don't put yourself and your kids at risk for a man that probably won't try and apologise or win you back even when he knows he's busted; instead just becoming violent.
I know you are hoping he won't go, that he'll come clean or beg for your forgiveness. It won't happen. He will go on his little holiday, get up to all kinds of mischief, then when he gets home he'll just spin you a story, gaslight you then leave his bags packed at the door like last time in an act of emotional manipulation.