Oh, I'm sorry he has lost weight after all. I don't think bunnies lose much weight when they're moulting? I've never noticed it with Sophie or Casper, at least. It sounds like a good idea to feed them their nuggets seperately to make sure he eats all of them. I wonder if it would be a better idea to keep him on the Junior nuggets since they probably have more protein than the Excel Adult ones and will help him more to gain weight? If you do put him back on Excel, do it slowly, yes. Feeding him more pellets might be helpful, too, yes, though as he was previously a bit too heavy, maybe feed him only a little more?
I'm not sure why he could've lost weight. Being bonded is stressful, of course, so maybe that's why? Or it could be his teeth, or some health problem. I would've thought that eating more veggies and less hay would actually make him gain weight, as veggies usually have more calories than hay, I think. Feeding the veggies seperately sounds like a good idea, too, though, so you can eat least see how much he's eating.
Edit: Oh, you edited your post while I was replying, heh.