I'm going to actually be serious and venture to say that it could be no more than half of his weight, and realistically closer to a quarter of it, for one such blow would primarily be delivered by the shoulders and arms with little in the way of hips or legs or leverage, and the mass of the arm muscles are fairly slight in comparison (50% of the body weight is in the legs if I'm not mistaken)
Of course, such a blow on the jaw may well be enough to hurt a similarly untrained opponent nonetheless. Beyond this, an untrained opponent wouldn't know how to position his body to redirect the force of a punch. They stand bolt-upright and wonder why they get knocked clean out when their knees, spine, neck etc are not braced to defuse the force of blows.
But it is actually impossible to give you a concrete answer in terms of kinetic energy or pounds per square inch or anything like that, without doing actual science. That would be a better question for Physics students, not boxing fans, lol