DS is a reasonably generous child, and we've taught him that it's nice to share and be shared with.
But I'm not going to teach him it's compulsory.
He's always been very careful with his stuff, and so as a result he has some fairly advanced and expensive lego sets, he's had an iPad since he was two, and he has some inexpensive but precious things that he values and takes care of.
These things are put away when certain friends visit, because they are not careful or generous themselves. The lego would be in bits and scattered everywhere and lost, the iPad would be in their not-so-careful possession and DS wouldn't get a look in, the other stuff wouldn't be looked after. In one case something was taken, either accidentally or purposefully. I've heard one of them telling DS he can't have a turn on his own iPad because "I'm the visitor and you've got to share so you can't have it" and I put a stop to that very quickly.
If other people's children are not capable of being as careful as DS, it's not up to him or me to teach them how to take care of his expensive or treasured things by forcing him to share them.
I've explained to DS that he can share if he wants to, and his nature is one that sharing comes naturally, but I've also explained that if someone else is not as careful or as kind or as gentle as he is, he can say no to sharing.
He also understands that other people may not want to share their own things with him in the same way. It's nice if they do, it's their choice if they don't want to.
I've learned the hard way that there's nothing wrong with refusing to share. I read a lot and have a large collection of books. None of them are collectors items but some of them are old, some are irreplaceable, all of them are important to me.
For the past few years I've flat out refused to lend books to anyone other than my mother (she takes care of them and gives them back) because of the state they come back in, if they come back at all. I've had them returned filthy, with coffee cup stains on the cover and great dirty smears of food and god knows what else inside them, pages torn or folded, or pages falling out. MIL has bloody well sold my books in the past and then been surprised that I've minded. To add insult to injury, she'd insisted on borrowing them in the first place, not read them, and then sold them because she was "fed up of the clutter" and they were just sitting about in her way.
I don't lend anyone a book anymore. I'll give a book away to you if I don't want it anymore, I'll buy you your own copy if I like you enough, but if it's a book I want to keep and you didn't give birth to me, you're not borrowing it from me. In return, I won't ask to borrow books from you either.
DS is probably my next exception. He's very careful with books, always has been even as a baby and toddler, and has been told he can read anything of mine he wants if he thinks he's able to understand it and can read it to himself. This arrangement came about when he was four and wanted me to read Doctor Sleep to him. I drew the line at reading my four year old Stephen King as a bedtime story but I mean it, when he feels ready and able to read it himself he can do it. Because I know he'll take care of it.
Sharing is important but so is learning how to respect your own and other people's possessions and appreciating that people have the right to keep things as they want them. And also learning that there are boundaries and you are not entitled to help yourself to anything/everything that belongs to other people, even if you ask nicely, nor are you obligated to let all and sundry loose on your own stuff if you don't want to.
It's thinking like that that causes the threads on here about borrowed money never being paid back or so-called friends expecting people to give them the earth for nothing (Bridezilla's, lift-sharers from hell, people borrowing stuff and then selling it on ebay, etc).
OP your DP needs to be a bit more generous with his own stuff and a bit less free at loaning out yours. He doesn't let his son set foot in his car?