Dear Jen this is such a difficult situation for you.
Verbal and emotional abuse is soul destroying. And it's very hard for a man to be on the receiving end. Particularly as the armed forces are supposed to convey a 'macho' image.
Just some thoughts:
Can he get support and help from the Families Officer?
Has he confided in a work colleague?
Can he speak to the Chaplain?
He needs to be able to 'off-load' in a safe, confidential place. I know that the Chaplaincy are used to dealing with domestic problems...one doesn't have to be religious to approach them. SSAFFA provide confidential support too.
You didn't say if your son lives on a base/service housing. In any case, a word with his NCO or someone in authority will help to steer him in the right direction. Whatever happens he's going to need support from the work, and it's not a sign of weakness on his part, it won't be seen as that.
Is your Dil is taking advantage of your kindness, don't be offended at my saying that, I know you are happy to help, especially as she is young. Being constantly in touch with you could be a genuine need for help. It could be manipulative and if that is the case then there is something troubling her.
Might she have post natal depression? She could be finding life very scary. That brings me to say...she needs to talk to someone too. Her Health Visitor would be a good start. Once again SSAFFA have workers that visit Mums and families. It sounds awful to say it, but the armed services need their personnel to function at full capacity, so it's in their interest to see that there is support available for families.
I know it's difficult as a parent to see your son so unhappy. I don't think it's good for the situation to be ignored. Help for them separately might clarify their situation, and give them tools to make the right decisions.
I hope this gives you some ideas on what to do. I experienced similar. Also I worked with armed service families until I retired.
Have a good chat with your son, let him know you are always there for him
[hug] 