You need to document everything you see. His behaviour towards his son, the off milk, anything he says or does to you. keep copies of your documentation and hide them somewhere. If you have a friend or family member you could send or email the information too, that would be good. Incase your husband finds it.
How long have you been living here? It can't be very long at all.
Sadly, I think even if you document, the court won't give you sole custody. But you could maybe get the right to make all the decisions! If you do, then you could take your son home. If there is no clause keeping your child here.
Just documbent everything which has gone on! And document everything you see which is happenng, when it does. If you keep a diary, it will help your cause I think.
What are you going to do if your husband fights with you and wins?
Personally, he sounds like a selfish jerk.
What about just leaving? Don't file anything. Just leave. Then you are still married and maybe nothing can be done? Take an extended holiday back home to America. I have a sister in England who was married and her husband took their child one day and then just didn't return her. The law said because they were still married, she couldn't do anything. She lost custody of her daughter to that man because of this. Not putting ideas into your head LOL. Don't know how things are here in this regard.
Unfortunately I think your ex has the upperhand, with this being his country. Go to a lawyer on the sly and get all the documentation and stuff done and then hand it to him if it is in your best interest. Leave while he's at work if you have to. He doesn't sound like a very nice person, so you do what you can behind his back before the !"#¤% hits the fan.
If you want, you can email me. I can't give any professional advice or anything, but I can be a shoulder for you

It sounds like you could use a friend (((hugs))).