A boy trundle bed has to have the guy touch and be sturdy enough to withstand the abuses of rowdy boys. Guys like to jump and play and have a big time on their beds. They are probably not as particular about a specific bedroom decor as girls might be.
However, the bed should be masculine and say something about how tough they are. Therefore, a traditional wooded bed or black iron bed would make a good choice.
Trundle beds for boys can come in handy in several different ways for practicality and play. First of all it makes a great place for friends or cousins to sleep when they come over to visit. They are especially useful if your kid has company over on a regular basis. They also prevent the stuffing of clothes and toys under the bed when it is room cleaning time! As far as the play goes, I can envision young boys pulling out the trundle and rolling off of the twin bed onto the mattress below. I can also see them bouncing from the bottom bed to the top and then down again. It could almost be like a multilevel trampoline! As a parent you may want to limit some of that activity, but you've got to let them have a little fun - and maybe they can use up some of their vast energy on their kids bed.