I'm so sorry you and your child are going through this. Obviously your DS is too young to know about the nuances, but I am sure he "feels" how differently your DH has been treating him.
I am adopted. My parents had one biological child and then found out they couldn't have any more (too much risk for my mother) so they adopted me. My father was absolutely wonderful - I was NEVER anything other than "his daughter". My older sister (his biological child) and I were both treated exactly the same in terms of being "his children". He was a loving father and in fact I ended up sharing more of his interests than my sister did, so my dad and I spent a lot of time together - fishing, fixing up cars, reading, etc.
Because of this, when I was younger I simply could not comprehend how some people could say they wanted to have children, but they ONLY wanted to have children with "their DNA". Adoption, fostering, donor, etc. were never even considered by these people - it was their biological child or nothing. It absolutely made no sense to me, because - to me, an adopted child is still your child. A child from a donor egg/sperm is still your child.
I am more understanding of how people feel now that I am older - sometimes people feel how they feel, even if I may personally think it is irrational.
That doesn't mean that your DH's current behavior is acceptable, however; only that I can empathize a little bit with how he may feel - he may not have really realized it fully, but he didn't want "a child", he wanted "his child" only. However, your DS is here now and IS his child, regardless of how your DH feels about the genetic/biological aspect of it, so that makes DH's behavior absolutely unacceptable.
Would he be willing to do any sort of counselling or talk to someone about how he feels? You can also try emphasizing to him - kindly - that DS is his child. His name is listed on the birth certificate. DS is legally his child - and more importantly, he is his child from an emotional aspect. DS has only ever known him as daddy. Perhaps try to gently emphasize that you understand how he may feel that DS is not genetically his, but he is a father now and needs to grow up and be responsible for his son. Sometimes the adults need to deal with their feelings and emotions in order to be proper parents for their children.
I personally feel that if he refuses counselling or continues on that he "hates" DS, you may need to consider divorce. If your DH does not deal with these insecurities and emotions now, he never will, and your DS will only pick up on them even more as he gets older. It is not fair to your DS to force him to live with a parent that hates him.