If your son will not listen to you or refuses to speak to you, then perhaps he may listen or be happy to speak to someone else, is there anyone you feel he would be ok talking to about this.
Sounds like this girl is very controlling and that spells danger, scary but true
Try and find a gentle way to speak to him. sometimes we have to try not to speak to our children like we are their Mothers, sounds crazy I know, but we have to drop the worry tone of voice to give some good advice.
I think you are right to be worried, I think you need to find an angle to get him to speak to someone, anyone, but this needs sorting, the quicker the better.
I know this sounds a bit off but perhaps his wife could even try and talk to him, she obviously loves him and hopefully she would be willing to help him out of this situation.
I think I would try and find out a bit about the girl, find out about her background, get some facts on her, it is so easy now a days if you have someone's name to do a bit of research on them. If you find anything alarming and you have FACTS about the girl, you could then perhaps present them to your son and that might help him to make a decision to get out of this dangerous relationship. I say dangerous because people that are controlling to that extent are dangerous people.
Your son needs help and you need to find a way to help, hope all the suggestions on here will give you food for thought, please keep us posted.