For purposes of this discussion, we are assuming you are choosing a bag for use in Troop 641.  We are located in Houston, so we generally do not experience weather much below freezing.  However, a scout in Troop 641 should expect a couple of nights of below freezing weather during his scouting career and should be prepared for it.  We have frequent rain.  Summers are hot and muggy.  Because of the wide extremes of weather, it may not make sense to look for a “year round” bag.  The better practice is to focus on a bag that will provide proper cold weather performance, then simply use a sheet, fleece blanket or sleeping bag liner in warm weather. 

Bag design and size.  Only a mummy type bag is going to adequately hold in body heat in cold weather.  Rectangular bags with open tops are fine for a slumber party, but when it gets cold, you need to be able to hold in all the heat you can.  Mummy type bags can be hard to get used to, especially for the claustrophobic.  Remember that you do not have to close the top unless the weather requires it.  Bigger bags are more comfortable, but have more volume that your body must keep heated and weigh more.  If you opt for a more expensive bag that is going to be a lifetime investment, make sure your scout will not outgrow it. 

Bag fill material.  Down is a wonderful fill material for a sleeping bag.  There is nothing as light, fluffy, warm and compressible as down.  Unfortunately, down is costly, difficult to properly clean and is absolutely worthless when wet.  This means that if you have a down bag, you (and your tentmates), have to be careful and disciplined not to get it wet and dirty.  One of the most common synthetic materials is polarguard  It is not as light, warm or compressible as down, but it retains its loft when wet and is more washable.  Another less common fill material is Thermolite, which claims to provide equal heat retention as other synthetics while requiring less loft.  This means that Thermolite bags can be fitted in a similar size stuff sack as a down bag.  They are also machine washable.  Having watched scouts for many years and having been one myself, I think the best option for a young scout is a synthetic bag.  Younger boys simple do not have the level of appreciation for their gear to take good care of it.  Even a careful boy is likely to share a tent with a boy who has not yet learned the proper respect for the property of others.  The chances that the boys will fail to properly secure their tent against rain or will let a bottle of water (or worse juice) leak in the tent and get their sleeping bag wet are fairly high.   Indeed, it is through these experiences, and the discomfort that arises from them, that young boys grow into young men and learn to be careful and appreciate their gear.  By the time they reach high school, most scouts are much better at caring for their gear.  For an older boy heading to Philmont, I recommend the investment in a down bag if he needs a new bag and you can afford it.  For a new scout, I would stick to a synthetic.  A moderately priced, lightweight and compact bag is the Coleman Exponent with Thermolite filling.  It is rated at 25 degrees.  My son has used one since he became a scout.  He has never complained of being cold, even though we have camped in freezing conditions on several occasions.  Sadly, this bag has been discontinued and is now hard to find.  I found a similar Thermolite bag at Academy and bought one for Northern Tier.  I still have and use my down bag that I got before my second trip to Philmont as a scout 30+ years ago, but decided I did not want a down bag on a canoe trip to a place where it can rain every day.  The new Thermolite bag is very compressible, lightweight and kept me quite toasty on those cool Canadian summer nights.  I will test it in colder conditions this winter, but from what I have seen so far, I think it is a winner.  For more discussion about fill options, here is a page I have found that details the pros and cons of a number of materials:  http://www.sierratradingpost.com/lp2/down-v-synthetic-guide.html.  Seeing all the products they have listed and all the modifications to the old standard products tells me there are a lot of options, not all of which I have seen, so don’t limit yourself to the fill choices I have listed here. 

I have never seen a bag at REI that was of poor quality.  If you buy a bag there, it will be more costly, but it probably can be trusted.  When you buy the cheaper bags at other outlets, you need to be careful to ensure that they have good zippers and sound construction.  Avoid the tendency to overbuy.  Don’t think you need a zero degree bag just to be on the safe side if you have no intention of camping in that type of weather.  You will pay more than you need to pay, and your son will carry the excess weight and bulk of this bag everywhere he backpacks.  My recommendation for people whose camping expectations do not extend beyond being in the troop, including Philmont in the summer, is to get a bag rated for temperatures no lower than 20 degrees.  If you find yourself taking a trip where the weather will be colder, you can generally get an additional 10 degrees of temperature rating by using a liner in the bag (and you can use the liner alone in the summer).