It sounds to me like your husband never really dealt with the death of his father, and perhaps the reality of his mom marrying someone else is bringing back all the grief he should have dealt with 4 years ago. Perhaps he's been in denial that he's really gone, but knowing that someone is "replacing" him is forcing him to deal with the fact that his dad isn't there anymore.
Grief is a very odd thing. I lost my grandmother almost 3 months ago. When she first died, I cried almost every day. I told myself I needed to just let myself be sad and that I'd get through the grief faster. After a few weeks it got better and I could think of her without crying. I was fine for almost 2 months, then over the weekend I had a run in with my dad over my step kids, and I cried for 2 hours non-stop. I finally realized that I was only partly crying about the fight with my dad, but I was more upset that I needed to talk to my grandma and she wasn't there.
I think your husband needs to see a grief counselor or join a grief support group. I think his anger is really sadness that he's having a hard time dealing with.