Does the typical dungeon world party need a good heavy-hitting class, or do you think he would be fine playing as a bard?
Good question. The answer hinges on how a party of any composition chooses to overcome complications. If a player wants to play a fighter, that is a pretty clear sign that someone is looking for a straight up fight. Players choosing to roll a druid, wizard, or thief might be more interested in slightly different solutions.
If you are assuming that the group is too scrawny, what does that say about your assumptions of the adventures they have yet to experience? If you foresee straight up fights that would be problematic without a fighter, perhaps some revision is in order to allow for other skill-sets to shine. Try to ensure there is always more than one way to solve a problem, even if the consequences of some solutions are more deterring than others.
If characters that might not be best suited to a straight up fight still decide to take it on the chin, wish them luck and play to see what happens. You might be surprised at how they not only survive but thrive.