The only new “trends” in the past few years with sleeping bags are: more accurate warmth ratings (standardized at least), more duck rather than goose as insulation, and most big manufacturers are coating the down with a polymer with supposedly enhances water resistance. I think costs are quite reasonable for what you can get.
For $300, you ought to have some nice choices. Your son will want a M’s<span style=”text-decoration: underline;”> long</span> bag for his height. I really like REI’s bags (the Radiant and Igneo), and the best bang for your buck is the Kelty Cosmic Down 20 at $170 for a long. It packs up to the size of a big football!
The nice thing about the new EN temperature rating system is that it puts the manufacturers on a level playing field as far as testing. The temperature rating on the bag should be a pretty good estimate of how cold that bag will be comfortable. In other words, a 20 degree bag should keep you warm down to 20 degrees. (Of course, everyone is different, so there is latitude for layering up in the bag, or even going with something not as warm if you run hot.)
Your son should have no trouble finding a bag that meets his criteria. Have fun!