For an adult bun the general guidance is roughly 1 eggcup of pellets (not muesli -science selective or excel are the most popular), 1-2 cups of fresh veg (a mixture from things like spring greens, herbs, pak choi, cavolo nero, kale, spinach etc, there are some good 'safe veg' lists in the diet section of the forum) and unlimited hay. Hay should be roughly 80% of their diet or around a pile the same size as themselves every day. This is the single most important thing about bunny diet - hay, hay, hay!
So I would aim to be feeding him that when he is around 6 months old, for now he can have more pellets but still loads of hay. And it is important to introduce new foods gradually, especially the veg as it can upset their tummies so I wouldn't introduce any more than one new veg per week and only in small amounts to begin with.