Personally I wouldn't bother with the junior burgess, it's even richer than the adult one, so if the adult one is giving him sticky poops, the junior one is likely to do so even more!
I would stick to a small quantity of pellets and loads and loads of hay. Do you have access to any forage? I have some 5-week old babies here who were taken away from mum too early (not by me!) and I'm feeding them science selective, hay and a good wadge of forage. I find that a forage based diet is incredibly gentle on bunny guts, I think because the more natural plants have not been engineered for maximum nutrition/soft fibre and have lots of nice tough fibre and aren't too rich. I'm feeding my little ones forage which is mainly willow, hawthorn, bramble, wild rose, apple leaves. When they arrived last Saturday the two littlest ones had no formed poos at all and weighed 430g. Now they do fantastic normal poos - I haven't seen a splat all week - and weigh over 500g. It really works! Obviously you need to introduce things slowly, I did it more quickly than I'd normally do due to their specific background and needs, but they are totally thriving. Might be worth a go to see if you can introduce a few bits and pieces to see if it helps.
Good luck with your little one