You are already on the right path. IMO, your issue may also involve the other parents spending $ on new equipment. It was with our troop. But if you plan it right, have a meeting with parents and scouts, like now, and if possible, show the alternatives in light/compact vs. what some may have, you can convert some. Bag, packs, and pad are your biggest bang for the buck, weight wise. We used campmor 20 bags, great value and small pack size, and BA pads. That cut a lot of space as well for some scouts in their pack. Don't underestimate the size of the food pick ups. 3/4 days of food is a major pile, and packs need to be big enough to get it all in them. What we did more or less is say the trip was X dollars, and depending on what your son's gear is like, the additional cost may be X more to get them the proper gear for a good hike.
FWIW, you may also want to buy some crew gear. We bought a 12' x 12' silnylon tarp that was great, way lighter and smaller than the Philmont one. It came in handy. Again, additional cost per scout that parents should be made aware of soon. Good planning, have fun.