I used to confide in my mother about everything. Then, as she got older, I censored what I said because I wanted to spare her any anxiety. Now , at the age of ninety, I tell her about the positive things in life and tit bits of harmless gossip. She has memory problems and easily gets confused, so I keep everything quite simple.
I miss the relationship we used to have, but accept that my mother now needs to be protected from other people's stress.
My MIL is five years younger than my mother. However, she has one or two health problems and has recently lost her confidence after a fall. However, my DH will phone her whenever he is stressed and pour out his worries to her. This includes every minor conflict he has with our youngest (17 year old) son. MIL will phone me, quite distressed, and I spend some time reassuring her that DH is just making a mountain out of a molehill.
It is really important that your DH is able to find other outlets for his anxiety. Our parents are often our closest confidants, but as they get older, their ability to cope with problems that they can do nothing about, can be reduced. Your DH also needs to know that some issues are private and confidential, even to his mother.
I know I am rambling, but I am also a mother and I relish the fact that my sons will confide in me. However, I know there will be a time when they will, naturally, be selective about what they choose to confide.