Please Help Me Ladies

Ok, I'm going to make this short as I can. My husband and I have a son together who is 2. My husband also has a child by another woman who is 7. Thing is....I feel as though he treats them differently. He treats his older son better. When I ask him to buy him something for our son he won't get it. But whenever his older son needs something he jumps at the drop of a dime. His child's mother already is getting child support anyway so it does frustrate me when he constantly buying him things. Then he feels like his son is an angel and can't do any wrong. 
Every time I tell him something about him that he does wrong he flips out on me. It's like I can't even tell him anything about his son without him being offended. But if I tell him anything about our son he is ready to spank him. It really makes me sick to my stomach and when I try and talk about it he thinks I'm comparing the them together and that I doesn't love his son. Smh. Idk what to do! Any advice?