As others have said he is most likely in shock since the change was almost the instant your DS was born.
I have always thought I was glad I was the one giving birth and not in my DH shoes as I would have found it very hard to watch something which is apparently so hard and painful for somebody I loved very much. There are very few other occasions when we are present in the medical treatment of somebody we love. It must be very very scary. It is also a question of control because he had absolutely no control over what was happening. You may feel that you didn't have any control over the birth process but at least you knew what it felt like and what was happening.
It is a good thing though that he has not withdrawn from you completely and is looking after you. I would have been personally more worried for you if he went into hiding and left you to it. He isn't going out with his mates or getting drunk every night. He is making an effort and I think, even if he is putting on a front for his parents now it could be that in time he will not need to pretend any more and the view he is giving to his parents is for real.
I am wondering if it might be a good thing that he goes back to work. He will have time to miss you and your DS and have a bit of breathing space and normality. Hopefully he will find himself wondering about your DS more and may begin to look forward to coming home. I do agree that you should leave your DH with your son as much as possible to give them a chance to bond.
However unfair you think he is being I think you need to remain calm with him. Try not to make him resent DS more by being the cause of a rift between you. It is horrible that you are having to juggle this especially when you are going through the hormonal upheaval. Hopefully in time things will improve although it might not be overnight.
I would also talk to midwife to see if she can either have a word with DH or to see if she has any experience of how to deal with this. She might not but it is worth a word.
I hope things sort themselves out for you all.
Congratulations on the birth of your baby boy too.