I find the chair vs. toy argument to be a bit of a stretch & unrealistic.
You are bringing TOYS to a playground full of KIDS. While I don't think it's okay for kids to just "swoop in" & play with your son's toys, I think it's downright weird to bring extra toys, when your son clearly can't play with all 10 of them, and tell other kids they can't play with them. We bring toys sometimes & I would never dream of telling a kid they couldn't play with them, even if they didn't ask. As long as no one's trying to keep anything, I don't see it as something to get worked up over.
IMO, the lack of wanting to share or being encouraged to share is going to hinder him socially. If your child won't share, your child most likely will have no one to play with. I wouldn't want my child to have that reputation, personally.
Why even go to the park if your kid just wants play alone, with HIS toys, anyway? I never understood that. It reminds me of the parents that will not leave their kids alone & hover over them the whole time they are there, hindering their child's opportunity to play & socialize with other kids & learn on their own.
Coming from the opposite spectrum, it also sucks to have a really friendly only child who just wants one kid to play with, and she gets completely ignored by rude, socially immature children on a regular basis. I just tell her to move on to someone that is nice & actually wants to play with other kids, and she's fine, but it breaks my heart a little to see it.
IMO, your child is more than old enough to get it. Is he in preschool or any activity that teaches him to share & interact with others?