if the old man is older than your father, then you call "bó fù", if he is younger than your father then you call "shū shu"

It is best to call him 伯伯 or 伯父 since he is the father of someone you know. Unlike the West, being older is an esteemed thing. Regardless if he is younger than your own father is immaterial. If a stranger, you can use 大叔 if he doesn't look to "old" or to be held in such high regard that it seems condescending.

I would definitely not call him "shū shu"or"bó fù" out of the dark like that. Without going into too much details, the safest approach would be to add lao3 before his last name, assuming you know it. He might then correct you with whatever he would like to be called and that would be just fine. Having said that, if you reckon it would lead to a global war due to cultural misunderstanding than perhaps it would be best to do your homework first as Ben wisely suggested.

Would it not be safer to ask that man's child, who is introducing you? Total strangers here are unlikely to give you better advice than your friend. Obviously you are taking this seriously. Don't trust anyone here, not because we are unreliable, but because there are a number of possibilities, and this matter of correct etiquette is fraught with pitfalls.

you can call him "shū shu"or"bó fù"