I bought a group of 45 degree bags for use along the VA section of the AT in summer. Best I did was a Mountain Hardwear Lamina 45. Also had good luck with Marmot's products - both a Pounder and a NanoWave.

A Mountain Hardware Lamina 20: http://www.campmor.com/Product___41201 might work for you, but is slightly more than you want to spend. However, at 3.1 lbs for a regular bag, he's not sacrificing much on the weight front compared to a down bag.

A Marmot Trestles 15: http://www.campmor.com/Product___42239 might also work. It is $70 cheaper but about 3/4 of a pound heavier than the Lamina 20.

I'm assuming that he's allergic to down or something else that prevents the use of down.

We're all assuming that your son's troop backpacks and that light weight is a criteria because he'll be backpacking. This is not necessarily true depending on the program your son's troop puts together. If weight isn't a criteria, the number of synthetic options available goes up, and their cost goes down. Also, the durability of the bags (and how long they'll last and stay warm) goes way up if they don't have to be crushed down into a little bag to fit in a pack. My wife and I both have blanket-style synthetic fill bags from the 1990s that are still warm, comfortable, and reliable. I wouldn't want to carry either bag far, but I've used the one for a trip before I wised up.

An alternative is a warm comforter over a foam pad. Using a quilt is something you can search about on this site. I've used the technique in summer with bedding I had about the house.