OMG, DH finally admitted, after four years,
What came to mind with your post Fresh, is that sometimes we can love someone, but not really like them.
I know this may not be easy, but I think it's good that your DH stated the truth. Now, the hard part will be to decide how you both should handle it.
IMHO, I would sit down with DH and tell him that he doesn't have to like my son, but he needs to treat him respectfully, regardless how my son treats him. There's no reason to treat someone badly "back".
"I feel such pain when I hear him talking badly to my son. I confronted him last night, and he said he was justified because my son wasn't respectful. "
That needs to stop. And I'd suggest having a talk with your son about either being more respectful, or both of them disengaging from one another unless they have to. Make the house peaceful where neither DS nor DH is speaking badly to one another. DH, being the adult, should be able to control himself and the situation better. You don't want "two kids" at each other's throats.
Hugs. Remember, don't force them to like on another, instead think of ways you all can live peacefully together (I don't care if it's FT or PT).
When I read your post, I thought well maybe it isn't your son he doesn't like (per se).
I have to agree with Cheryl 100%.
The choices we make in thought, word and deed inevitably return to us in kind.