Cherry, try not to worry about wider things than you need to. Your S is fine, and it is what it is for now. This is happening in his first year, and there's not a lot you can do to change that right now. Focus on what you can control.
Have you had any further contact with H since you posted earlier? I think if you haven't - and if you can manage it - you are best to have as calm an evening as possible. If you can, try to act 'as if' you aren't sitting waiting to hear what he has chosen to do.
You have already made your choice and told him. You won't be around waiting for him if he chooses to be with someone else. All you need to do is maintain that boundary and he will do what he will do.
Think about the worst that can happen. He may come back or contact you and tell you he wants to be with OW. Okay, that would be hard, but you would get through that. He may move out if that happens. Okay, you would cope with that too. in a way, you're already coping with all of that.
In a way the worst has already happened. And whatever decision he makes today, it will just be a bend in the road in your sitch and you will see that as more time passes.
He will do what he will do right now. There's not much you can do about that - so you might as well calmly go about your own business and let him go if that is what he wants to do. If your sitch goes like most here, he will soon find the grass is not greener as he thought.
Take care and keep posting Cherry x
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus