Generally a 5 yr old does not like a new puppy due to the pup being very active and wanting to play. Also, he has been an only child for 5 years too and in turn does not understand why this dog is there taking his loving from his mommy. You have to be prepared to put a LOT of effort into helping your boy accept the new pup. This can take months. Be careful and not let him hurt her. I had this issue too when my oldest was 2 years old when I brought in a new pup. I swear I think my girl thought she was human and did not understand why we bought a pet. Good luck
