As cute as it sounds... in this day and age, it sounds more like a pet's name than a child's name. However, my husband is constantly saying, "Hey
Buddy" to our 15 month old son... I do not consider it his nn as there are other nns we call him, but rather a term of endearment. As I was writing this... a light just went on for me regarding
Bud... it sounds like a term of endearment rather than a nn or fn... if that makes any sense.
I don't understand why, if your SIL opposes this name as a fn, if he wouldn't mind your daughter calling her son
Bud as a nn. Would he have a problem with that? It may not seem like a natural nn now, but in time when you start to wrestle and play with the boy... it does seem rather natural to call him that... I find myself doing the same thing with our son now that he is more active... but it would have seemed too odd while he was just a baby.