A Talmud chachom shouldn't serve his son?!

Kiddushin( 31b): R. Jacob b. Abbahu asked Abaye: ‘I, for instance, for whom my father pours out a cup [of wine] and my mother mixes it on my returning from the school, what am I to do’? — ‘Accept it from your mother,’ he replied: ‘but not from your father; for since he is a scholar, he may feel affronted.’

This is the halacha in Shulchan Aruch (Y.D. 240:25)

Thus that the halacha is that a father can serve his son a drink - but not if the father is a talmid chachom because it is against the mitzva to honor his father. Is this a din only for a son or it applies to every talmid chachom? Why will the father be upset if his son accepts? Why should the father even offer it if it in fact will cause him to be upset if his offer is accepted? Why wasn't Avraham upset at serving what he though were idol worshipers and in fact he is held up as a role model of chesed?