While I can't fault the logic of recommending a synthetic bag for this purpose, for my own son I bought a -12 down bag (~1.6 kg, 10.3 L packed ... it's 575 duck down) and he's used it succesfully thus far (though he's no longer in Scouts). My daughter has a -20 hybrid which is an interesting combination of down & synthetic fill (~2.2 kg, 15.4 L packed) ... its still really big for my likings but Mom insisted on a -20 bag (but I don't have a good compression sack for it either). Both options run a bit over $200 (the linked bags are updated versions so the numbers might be a bit off for what we actually have).

If you were in Canada, I'd recommend a MEC bag without hesitation; I think they are a great value.