I really agree totally with what the other comments have said but I think it might benefit you if you got a behaviourist to help you as if your not confident in what your doing with the dog the situation could get a lot worse. Ask your vet for a recommendation. Good luck, remember stay calm as the dog will feed off your energy.

I agree with what was said above, it sounds as if your dog is starting to believe HE is the pack leader. If this is so it is because you have allowed him to think this. You must constantly re-assert YOUR leadership, by many little gestures, you eat first, you go out of a door first - stop for a matter of a couple of seconds before inviting him to follow you, you own the territory, (have places where he is only allowed to go if you invite him), when you walk him on the lead always keep him beside or behind you, NEVER in front. Dogs are very simple and straightforward - the one who is in front, who controls the food, who eats first etc is the leader and amongst themselves the pack leader constantly reasserts his authority by these little gestures. When you have a family pack of dogs, as I do, you see these things all the time if you watch their behaviour. Of course most puppies get taken away from their family far too young, before they are properly socialised, and then, being the cute new family puppy, get spoiled rotten and grow too big for their boots. Sounds like this is what has happened with yours, but with patience you can restore the balance.

You need to get on top of this NOW before it gets worse. You need to make sure YOU are in charge in every situation so he knows you're the 'top dog' in the pack. You can start to do this by not allowing him up on furniture, only feeding him after you've eaten and starting some basic obedience training. Particularly if he is a male, I would highly recommend neutering him. Can make a big difference when it comes to aggression. Don't let the problem go on too long. Get advice from your vet and see if they can refer you to a behavioural specialist. And most importantly be careful around him. For now, don't challenge him around food, etc. It's not worth the risk and be especially careful if there are kids in the house.