How big of an issue is favoritism ?

I have a son just turned 8. And my husbands daughter is 9. We have been together since my son was 2 and she was 3. So along time. I have noticed that over the last probably 3 years my husband has been treating them differently. He seems to yell at my son constantly for nothing really and never yells at his daughter. He loves on his daughter a lot and never shows my son attention at all. He calls my son annoying all the time. But has never said that to her. His daughter is more mature than my son. And my son is just hyper and likes to be doing stuff all the time. But he’s not bad, he’s very polite. My husband always says stuff like he needs to be more like his daughter. And says my son is horrible all the time. (Not true). And my son over hears him and cries. My son use to call him dad but recently has stopped and calls him by his name now. My son has also mentioned that he thinks his step dad doesn’t like him. It breaks my heart. I have talked to my husband about it before and he tells me I’m insane and that he loves my son and that I’m over reacting. If my son cries after getting hurt or in trouble my husband mocks him and calls him a baby. Again doesn’t do this to her.

What should I do ?