My husband is constantly picking on my son and I don't understand why. Why do men have problems with their step sons? My husband is 55 and grumpy my son is 31, a christian and always very pleasant.
Sounds like its an immature selfish thing your husband has going on inside himself.
There are just too many reasons to mention ... It would depend on what kind of relationship they have cultivated. I would dare to say it is imbalanced .. At best.
They may simply lack comunicational skills (at least towards each other) that could help them both get AND give what they need to in a step-father and step-son dynamic. They are both adults now .. And perhaps a family discussion about their dysfunction in your home is long overdue.