How much does he weigh and what species of dwarf is he?... I ask because I've fed burgess dwarf in the past (about 4 years ago now) and the winter white I had at the time was grossly overweight. I've also noticed the same issue (overweight dwarfs being fed burgess) with others too. The food is really poor quality - and it's really not a shame that it's going; it's pretty much straight up millet, and just a very poorly balanced diet. So the weight he is losing could actually be a good sign, but it's really impossible to tell without knowing how much he weighs and what species he is. Regardless; 5g is really not a lot - certainly nothing to be concerned about given that it was lost gradually like you say over a few weeks, so it likely was weight that he could afford (and needed to) lose.
I would stay away from all pelleted brands (burgess, harry hamster, pretty much anything you're gonna find at the local pet store etc), personally. They're *always* low quality & poorly balanced diets. Generally speaking, if you want a high quality, species appropriate diet you need to order online. The bunny expert foods are really nice, and you could keep going with that if you'd prefer; other suggestions I'd have would be rodipet, who have a good hybrid dwarf option (I don't recommend their 'normal' dwarf mix - the hybrid foods only) and my personal favourite is getzoo. I have 4 WW's currently; they all adore the getzoo mix, and they're all very nice weights too for their species. I've never had any problems switching them onto it when they come to me either, they all love it!