Way, way back, around the dawn of time when I had the time, I hand-raised a couple of litters of wild bunnies on homemade formula based on condensed cow's milk. I was just trying to get them old enough to be on their own, so I wasn't terribly concerned with weight gain; as soon as they started eating good quantities of solid food, I stopped hand-feeding (wild rabbits wean very young).
These days, I simply don't have the time; I breed my does in sets, so I almost always have does that can take care of the babies whose mothers can't or won't look after them. I have had a couple of litters that were orphaned at about 3 weeks when their mothers died. If I had had calf manna, I'd have given them that, but since I didn't have any on hand, I just gave them hay, pellets, and rolled oats. They had had a really good start while their mothers were alive (in one case, perhaps too good a start; I think the doe died of a calcium deficiency), and they continued to grow as well as the other litters that I had that were the same age. A lot of commercial growers wean at 3 - 4 weeks; you don't have to hand feed for 8 weeks, but if this bunny is undersized for his age, he probably does need the extra calories.