It’s definitely possible to have a happy, healthy singleton. You say that your rabbit seems lonely. Is he acting out? Moping? Destructive? All of these things can be a sad bunny. I think the most important thing for single rabbits is companionship with their owners. We are the only social interaction they have. It is vital to spend as much time as possible with your bunny. I work during the day but let my rabbit out to play in the morning and as soon as I get home at night. There are exceptions if something comes up, but I try to give him as much playtime as will fit into my schedule.
Also, your rabbit might just be bored. Make sure to have plenty of interactive toys that you can rotate so they will always be exciting. Mine really enjoys toilet paper rolls cut so there is is fringe on the ends to toss around. I also fill them with hay for snacking. Baby keys, sea grass mats, and wood chews are all great options. I hope your rabbit feels happier soon.