Seventhheaven, why put on such a brave face when surely your son will expect you to be upset, as any loving mother would be? It's okay for you to be happy for him and his wife but sad for yourself. That's entirely normal. By pretending otherwise you even risk appearing not to care.
Mightn't he even be doing the same as you? Agonising over hurting you but keeping his feelings to himself because it's the "done thing" in your family? You and his wife are no doubt the two most important women in his life, but in this situation he can't give both of you what you want. He has to make a choice. He has to move on.
Be brave and get it out in the open. Talk to him about it, and don't be afraid to cry! You will probably make it easier for him by telling him what I'm sure he already knows - that of course, for your own sake, you don't want him to go, but that you would be even more devastated if you thought you'd done anything to stop him and his wife pursuing their dream.
As you can see, there are lots of us in the same boat. Me included. And now I feel myself filling up as I write this!