I have two adult daughters. When I told them that I was going to transition, one of them began to cry. I asked her why, and she said that she was happy I had finally found the source of my anger. Our relationship improved quite a bit since then. The other daughter didn't have much of a reaction, but our relationship had been reasonably good for several years. My wife was also quite upset, but she eventually realized that I was becoming a better version of the person she fell in love with all those years ago. We are still together.
I think the key is just to give them space. Let them know you will answer any questions they may have, but don't be insistent. I think the longer someone has known you, the longer it may take for them to adjust. Unfortunately, not everyone will be accepting. Even family members may reject you. As long as you give them time and space, whatever decision they make is not about you, it's about how they see their world.
Separating from your wife could also be part of the issue. The family your son has known is changing radically, and that can be a stressful situation no matter how old they are. It may take years for the dust to settle. Gently keep your foot in the door, and let him know you are there for him.
Love always -- Jessica Rose