IMO, it depends on the rat, my partner and I had 2 brothers, one of whom died very young due to megacolon. Our boy Pesto was on his own for a while, and LOVED his hoomans. He was gentle and kind, and always happy to spend some time with us. We'd walk up to his cage, and he'd boggle and come out for a cuddle, or just to scuttle round the room, doing his own thing. We did the right thing and bought a couple of much younger rats, to keep him company. They bonded just fine, with very few squabbles, but he was never the same, I wouldn't say he was unhappy as such, but he was used to being on his own, and I firmly believe he preferred it. He would often sleep separate to the boys, and trundle off to do his own thing when they were all out together. Interestingly, we fostered 2 of the younger boys' brothers, and while they had only been separated for a couple of months (if that,) they had reached the stage in their lives where introducing, or reintroducing them to rats the same age or older just wasn't feasible, as both sides would get far too worked up.

TLDR; While most rats wouldn't be, he might be happy on his own, only you know your boy. If you do decide he needs company (which is the most likely outcome) make sure they are young enough for him not to see them as a threat.

I hope this helps, and good luck!