Is this harmful in any way? He can't pull back his foreskin but he can masturbate, it doesn't hurt he tells me. I'm really worried
Not all males enter puberty by 13. It can often take until 15/16. If no signs after 16, he needs to see his doctor.
Does he have any signs of puberty?
Most guys can't retract their foreskin until about age 16 or so.
He CAN and SHOULD be stretching it. Tell him to get an erection and pull down on the shaft skin OR use his fingers to spread the opening. He may find it easier to do in the shower, the warm water will help soften the skin.
The more he does it, the sooner he'll notice results. He will likely find his glans very sensitive, almost painful, at first. This is normal and he'll get used to it.
Congrats on having a great relationship with your son. Good luck.
I want to reinforce everything Dan said especially the congrats on your relationship. It must be really nice for him having a mother he can share this with and who wants to help him.