Mellors, another experience...

My middle son found out his wife was seeing someone else while he visited here for a week. Her affair developed, my son moved out, the "other man" moved in permanently. My granddaughter confronted her mother herself! She said to her "Why isn't Daddy living with us anymore? Why is "X" living here and sleeping in your bed? That's wrong!" She's the same age as your son. I DO believe that your son can take the truth in a small dose. Maybe if you tell him that his mother didn't love you anymore and found someone else. No judgement of her or her actions. Just the plain truth.

When you do see or talk to him, reinforce how badly you miss him and tell him the times you've tried to talk to or see him and weren't able to because his mother didn't allow it. Just mention the times and nothing about his mother. Just state the facts and let him make up his own mind. He already knows his mother won't let HIM initiate contact. Kids are sharper than we tend to give them credit for. Doesn't seem to me like it will take him any time to make that judgement call for himself.

Something "proactive" you can do is check with an attorney/courts to see what can be done to enforce your visitation rights. If you can afford it, make the trip down there and knock on his door. Let things take their course. He'll know you were there trying to see him and he'll also know who it was that stopped you.