In touch with his ex..?

You seem very confident in your partner and that's a good thing. He has obviously gained your trust during the four months you've been together, and I can definitely understand what something like this would do to you. It's just that him hiding something like this from you is a very big deal. Him giving her money on each occasion is even more peculiar, whether or not it's her birthday or her sons. I would feel very uneasy about this. That's time he could have spent with you or money he could have used to take you out. Just because he met up with her doesn't mean they did anything sexual, but I wouldn't completely rule that out if I were you. I could kind of understand if they just wanted to keep their friendship, but when he's doing this behind your back it just seems to me like he has something to hide. And, he said he didn't tell you because he didn't want you to worry. Well, obviously something is going on that you should worry about. It's your choice if you want to see where this relationship goes, but the longer you're together with this person the stronger your feelings will grow and the harder it will be to leave if something like this happens again. If you truly believe he is being faithful and you care about him and he treats you good then I would stay with him. Just follow your intuition because that's what I have done and I've always been right whenever I listen to my heart.