Boys tend to like more:
graphic/geometric prints
sports
bugs
cars
cowboys
fishing
bacon print
I'd avoid:
florals
swirly or paisley prints
cutesy Disney-type animals - go more for realistic deer, bear, moose, etc.
For a while I worked with a group that made quilts for terminally ill patients at one of our hospitals. I'll never forget one little boy who picked out a quilt for his mother, who was dying of breast cancer (heart breaking!!) He picked a quilt that had apples in it, because she liked apples. So there's another thought.