i've been through what you are going through and you really do have to find the strength to sort this out, before somebody else steps in to sort it out for you!
My dp is also from a different culture, and was tired and stressed with work, and looking after four young children, etc. etc. One of our children has sn and dp has always found him particularly difficult to deal with.
I didn't realise how bad things were between dp and ds1 until ds1 confided to his teachers at preschool and social services became involved. A lot of things had happened behind my back. I'd be worried too, about what your dh may be doing when he has sole charge of the children.
My dp isn't a bad person, and I don't think your dh is neither. It's lack of knowledge, he needs to be taught how to deal with his stress and his children. It doesn't come naturally to everyone, especially if their upbringing was very different to what is acceptable by western standards.
If social services get involved, your dh could face criminal charges. He may be forced out of the family home. He could be required to go onto parenting courses whether or not he wants to. You will have the intrusion of them investigating every aspect of your lives. You need to take action now to sort it before it gets to that stage.
The parenting course that my dp went on was actualyl run by a christian organisation, it is worth looking around to see what you can find.
Your son needs to know he can rely on you. It broke my heart when I discovered that my son hadn't felt able to tell me how bad things were for him, and things that had happened behind my back. He is so small, he needs you to support himm and listen to him.
You an work through this, but you do have to put your son first and you need to lay down some ultimatums to your dp and you an't be worrying about him leaving for his family., Your son omes first! Your dp needs to go on a parenting ourse whether he likes it or not. It might even be worth onsidering family ounselling too, so that you an find a middle ground and parent effetively together.
please dont leave it too late, like I did. We went through a horrendous time with soial servies in our lives and although it was what we needed to get our family in order I wouldn't wish it on anyone.