Just keep making sure he is getting hay or grass (I can't quite remember what you said you are able to feed him) and making sure he's getting whatever kind of roughage you can grow or find for him. It may take time for him to put on weight. Pellets too would be a good way to get weight on him; they offer a lot of good nutrients for rabbits too, although I would feed in moderation. For pets they don't need many. Both my rabbits together get half a cup a day.Bugs is weak. I feel his spine it's really pointy. I fees him a mix of carrots apples cucumber corn husk new grass sprouting in our garden and I'm growing wheat grass for him too. Wht can i do to make him healthier I cant find many leafy greens except spinach here they are hard to find. People don't buy them.
I also believe some grains in small amounts are okay for rabbits, although I'm uncertain what would put weight on him. I'd ask around on here or do some research. For a pet, I'd keep away from grains as general food unless you are using them for treats or specific purposes like trying to put on weight.
How big is he? Another thing is to either get used to how heavy he feels, or if you are able weigh him. That way you can gauge his weight too. You can still just feel bones like their spine and leg bones when they are a healthy weight, but not overly much.
I hope he puts weight on soon!