LIF - It is always darkest before the dawn. He needs to come back because he has decided that is what HE wants to do. Anything less will be unsatisfactory FOR YOU. Sometimes people get into something and pride, selfishness etc., get in the way of doing what makes sense. He needs to miss YOU and who you are. He needs to miss S4 and to miss out on what your "family" means to him, and that you get a whole different dynamic when you split that apart.
What are the chances that this other R will work out? Do you think he can really live as a married man, with a married woman for a whole year? Has he changed from the person you fell in love with to a person who can really do all of these things?
In the meantime, you will have to fill the gap for S4. I know MY S4 (just turned 4 Sept. 30th), knows when "daddy" has been on business trips for a long time. He tells me he misses him and gets whiney. You will have to be strong for you and for S4 right now.
H will have to come back unless he plans on abandoning S4. Perhaps when he realizes how much he is missing of his sons life, it will help him reconsider.
From here you will grown and get stronger. WHEN he comes back, you will have to sit and think about whether you want HIM back. (The answer should be yes, even though it will mean lots of hard work to rebuild the love and trust you have lost, b/c you will still have child that deserves a happy/healthy family.)
Keep posting, keep venting, and we will try to keep your spirits up throught this journey. We did not ask to be here, yet here we are, supporting each other. That is a blessing in and of itself. - M