Down bags are not good because they are ineffective when wet.
The sleeping bag should have a comfort rating from 20° to 30° and should weigh no more than 3.5 pounds, but preferably less.
A couple of large garbage bags may be suitable for summer hiking in the Sierras, but if multiple rainy days are anticipated a dedicated pack cover is better.
The conventional wisdom used to be that external frames are for trail hiking and internal frames are for off trail hiking.
Most of our hiking is on trails, but the majority of Scouts and adult leaders prefer the internal frame and these have become the most commonly available packs.
Sleeping Pad & Pillow: A closed-cell type foam pad (e.g., insolite or the accordion style by Z-Rest work well) is good to keep out the cold and to preserve the sleeping bag.
Shorter length pads can be purchased, or longer pads can be cut, to reduce the amount of bulk and weight (head to knee length is sufficient).
There is lots of discussion about the proper amount of weight that a boy can carry safely.