He then blew up and said that he can't explain it.
Many of us men are not well practiced at identifying and interpreting our emotions. For this reason, many emotions and feelings (e.g. fear, embarrassment, shame, and frustration) express as anger. I do not write this to excuse your WH’s behavior, but to point that this is one of the many things a WS needs to improve after dday.
He said that he was frustrated with himself because he is a pathetic loser and he doesn't have a different answer for me. He sai he wishes he did so I wouldn't be in so much pain.
If he is in IC, they should be working on helping him to identify and express his feelings as well as overall communication. This is especially true if he is socially awkward and worse than average at inter-personal communication. Improved communication skills will help not only in his personal relationships, but at work too. If he is not in IC, this is a sign that he really would benefit from it. These skills can also be improved by reading about communication and then practicing.
BTW, he needs to lose the “pathetic looser” bit as an excuse and be a man if he wants his M to work. I doubt you want to be M’d to a pathetic looser. It may be true, but he needs to own what he did, figure out why, then put in place and practice behaviors to prevent re-occurrence.
As for the apology, I had a teacher in school that used to say “if you’re sorry, change”. Saying I am sorry is cheap and easy. Apologizing to acknowledge a fault, and then working to change behaviors or awareness to prevent future occurrences is much harder, but demonstrates true remorse.