He doesn't have anyone who has taken (or is willing to take) the place of his mother.
I have to agree with him that having a day for mothers (and fathers for that matter) is worse than useless (and it's biased that there's no Sons' Day or Daughters' Day). It's a horrible day for those people whose mother is dead, missing or horrible. Likewise for women who've long wanted to have a child but have been unable to, and for mothers whose only child is dead, missing or estranged from them. For anyone who has a living, good mother - surely they don't need one day a year to be reminded to see her. If someone doesn't bother with their mother for the other 364 days a year, then she's clearly not important to him/her. The only winners are florists, card shops etc.
Does anyone have any advice about how he can make it bearable? It's not just Mothers' Day itself, it's all the talk about it during the days before and after. On the first Mothers' Day after her death, some people told him that it won't be anywhere near as bad during future years. However, he's found it just as bad each year. Time has not been a healer for him at all.