Is my guinea pig happy?

I've never made an account before because I had no need to post, so I don't have enough karma to post this in other subreddits. Hopefully it'll be okay here.

Hello!

I'm a first time guinea pig owner. I've had my little guy for about seven months now and I'm trying to do right by him. I work at an animal shelter and there was a guinea pig in a small cage that no one was putting applications in for. I ended up taking him home because it was breaking my heart. I don't know too much about where he came from, just that he was abandoned in an apartment and the landlord eventually found him and his dead friend.

At first I really wanted to get him a new friend since I know they're super social, but he's been aggressive with any I've tried to introduce him to. He ignores them mostly when they're separated by a cage, but he bares his teeth and lunges when they're put together. I've tried taking the introductions slowly, but I can only have another for so long before having to commit to adopting them.

So my guinea pig has been alone for about a year now. Seven with me, three at the shelter, and however long his friend was dead for. Or maybe he didn't like that pig either and they fought? I don't know.

He does not popcorn ever, or at least not that I've seen. He wheeks during food time or sometimes when I walk into the room. Sometimes he sees me get into bed and starts, so I usually go get him and cuddle with him on the bed. He never makes any sort of content noises. When I hold him he stays completely still. When I'm laying down and he's in the crook of my arm, he'll just sit there. Sometimes he'll pick his head up and sniff, but then he'll go back to how he was. He's not properly laying down, just sitting there completely still. Sometimes he snuggles in a bit, I think, but it's hard to tell when I can't move to look at him. He likes to be under my chin so it's hard to observe his behavior.

I don't know how to tell if he wants to go back. He'll chitter his teeth when I pet him for too long, but he's entirely silent when I'm holding or cuddling him.

While he doesn't popcorn, he's very zoomie. He likes to sprint all over his cage and my room. He also likes to nibble my toes and chew things he shouldn't be. He does not play with any of the toys I get him unless he can eat them. He sometimes sleeps in his hides or beds, but he prefers sprawling out on his fleece bedding right next to the bars. He has a healthy appetite and drinks lots of water.

Is there something more I should be doing? Is he okay? Am I misreading any of his attempts to communicate?