Hello,
It sounds as if he is challenging your authority a little, not unusual for dogs this age. It may be worthwhile to put him on the lead when visitors come and tell hime to sit, reward him with a treat if he's stays sitting but try not to make to much fuss of him. This is something that can be overcome with a little training, it will take time but it's not impossibe.
Barking is usually done for attention and I think it's probably best what you are doing in ignoring him until he's quiet.
It may be best to speak to your vet about the neutering suggestion, I think that all puppy's go through this age and that it's completely normal and they do grow out of it, I know mine have!
Hth