My bun has been a loner his whole life, and has been extremely happy, but I've dealt with this in rats, who are also super social. Give him some time, he's certainly grieving, and probably just needs your attention and love just like you need some in this hard time. If you're ready for another bun, start looking and preparing for intros, but if not, just let the process happen with some extra love and attention.
At some point, we sometimes decide no new animals should come into the home for whatever reasons, I'm at that point after my 5 years of caring for rats. I've seen 10 of my buddies die in the last 2 years because of their short life span, and the loss is very hard on me every time. I currently only have one rat, which is generally a big no-no, the same with rabbits. But if you're willing to step up care and affection, don't worry, he'll bounce back with some TLC.
My boy has been alone his whole life, I adopted him and he has been alone since birth, weened too early. He has very bad rabbit aggression, but gets along with all other creatures including my rats, and has lived a very social, happy life being a lone guy.