Denial of anger, especially toward a parent, is not uncommon. There is a little test of forgiveness, the Personal Forgiveness Scale, in Appendix C of the book, The Forgiving Life. You might want to ask your husband to fill this out first on his employer (as a warm-up to familiarize himself with the scale). Then ask him to fill it out toward his mother as he thinks of one incident that he deems as unfair from the past. The explanation of the scores is in Chapter 9, starting on page 156. If he scores between 18 and 63, he likely has some forgiveness work to do. He should then consider doing some of the work in Chapter 10 of that book.