this summer i took my 5yo "groundhog hunting" with me with a loaded .22 rifle. the responsibility of the gun was mine alone, and i made sure he knew the rules before we even left our home. he was very attentive, and followed the rules i gave him. we didnt see a single groundhog, but he hung on my every word about what we were doing and why; what to watch for, why i handled the gun the way i did, what the gun was capable of--both good and bad, and just nature in general. it was probably the best afternoon of my life! i understand why your dad took him.
oh, and my son is ADHD and a complete daddy's boy. he emulates me so completely that good, serious, proper teaching about something that his hero loves, is more than enough to overcome the ADHD. a responsible adult teaching a young one who looks up to them will blaze the best trails; teachers at school can't even compare.
while i agree that grandpa should have your permission to do anything with your child (i reserve that right with my parents, too), i don't think it was wrong of him UNLESS you wanted to be the one to introduce your son to hunting or grandpa has old-age issues, etc. as for the young age, believe it or not, the younger they get started, the better the safety rules and nature lessons stick with them. that's how i learned. it sounds as though your son was excited about it!
i hope i'm not out of line, but if it's a safety issue, i don't think your dad wants anything to happen to his grandson. go easy on him.