Knowing that your FIL doesnt live with you, first time over, and he didnt actually bite him at all, I think either he thought it might have been his "dad" for a second, etc. Aggression is one of those things- you do not just look at the situation, you also look at when it doesnt. There is a common thread in there- and it could be he was suddenly surpised and confused for a second, not given enough to figure this out for himself- then put in a situation where he had to make a decission. With Rockys' size- he easily could have hurt him- Rocky choose not too- and told him to back up a minute while he decided it was okay. Either meet outside of where he thinks is his- ie on the walk way for a short walk with you, the man, and rocky, or just give Rocky more time to adjust and do not let someone pet him until Rocky is ready.