So yield, show respect for the man, learn some of your own humility and let him train with no bowing. Just a nod will cut it for all japanese I know. But we want to know better, isn't it.
This is certainly a very interesting question...
I'm all for being humble, and understanding of others - they are among the key features of aikido, for me; the question for me is whether a disparity between two people can - or should be - tolerated: you accommodate them, they do not accommodate you - the relationship is thus asymmetrical.
I regard bowing to one another - regardless of rank - as an act which symbolises equality (as well as gratitude): if one person refuses to bow to another, therefore, there is the implication that they are superior to them.
I think we've become quite abstract in this discussino, because we do not know what the individual's views are: does he object to bowing because he will only bow to his divine being? Does he just need to be told of the cultural significance of bowing in Japan - i.e., it is to show respect, rather than supernatural reverence...? etc.