At his age, being retired and seemingly having grown old and frail (75-79 years old) I highly doubt he can take more than 2 big attacks from Luffy or run around playing tag more than 10 minutes. However it doesn't stop him from being able to have all of the old man techniques (Neptune, Hyogoro, Rayleigh, Gan Fall, Lao G).
If he could teach Sanji anything apart from haki I would love for him to teach him some axe technique.
Sanji should be somewhat of a natural with sharp weapons and it would automatically boost his attack power by 50 %. All of a sudden he would have sharp weapons comparable to swords for offense and defense, he would be able to use his upper body without using his hands directly and he would add ranged attacks to Sanji - the only character I know off that doesnt have any after 1000+ chapters.