1)You should try assisting him when you *catch* him masturbating. Normal or not there is nothing wrong with masturbation.
2) If he lets you know he doesn't want you to assist him or resists you ask him why. And keep an open mind about what he says.
Age and childbirth along with other things can make a woman lose vaginal elasticity (tightness of her vaginal walls) over the years. He may want a tighter grip, in that case kegals can give you muscle control and there would be nothing tighter than you if you can master that skill.
If its lubrication you need mroe foreplay or perhaps store-bought lube.
It may just be him trying his best to stay *up* as long as possible ( my husband did this when we first stopped using condoms right after he made me orgasm several times he's masturbate to make himself cum and after a few tries of this reached a point where he could orgasm/ejaculate inside of me and remain stiff, sex is AWESOME after that point the possibilities are endless) or trying to avoid fatherhood. But you can rest assured his sexual appetite has not left.
This can be worked out, don't feel bad!