I remember in my day ( alooong time ago) hip belts and frames were all the range but I suspect things have moved on after two decades? Plus he'sonly eight, and a small eight at that. yet the cubs note says kit rucksack must be able to be carried by ds2.
Depends how big his sleeping bag packs down to, I remember taking one of those huge square foam things when I was wee and it took up half my 50L sack!! As he is only small its a good idea to stick with a small pack as then there is less room for c*p and will encourage him to pack light! will he actually be walking carrying overnight stuff, or will they be based somewhere and he will use it for day walks, eight seems little for backpacking when you have to carry it all yourself!
30L is quite a lot, my adult one is 80L and I have packed all I needed for an 8 week trip in that! Plus to be honest he wouldn't be able to carry much more. If his sleeping bag is a decent make of mummy style (and child specific) it will pack down small and light then just role matt, change of clothes and washbag and towel... should be fine