How long have you had him? I ask because 6 weeks is usually a bit young for a chi to be away from his/her momma. His agression could have something to do with that if he was removed from momma too early and didn't receive proper socialization from her.
Also, you might want to mention his aggression the next time you're at the vet's office. He could be tender around his belly or have some bloating that makes it uncomfortable to be picked up.
Most dogs that "attack" in the manner you're describing are acting out of Fear Aggression. Seeing as how he's so young, he may not feel comfortable in your home yet and picking him up frightens him. One of my training books says to avoid rewarding this behavior by ignoring it. Do not punish him for the aggression as it will frighten him, making him even more aggressive.
Additionally, it says to avoid playing aggressive games with the dog like tug-of-war and other little games were you're encouraging the dog to be aggressive. Of course, he's a puppy and he's going to have a little aggressive play but you might want to watch how much he's getting in.
If the pup is sleeping in your room with you or in your bed, you may want to try crate training. It's possible he's trying to gain dominance, although I don't know if he really would be at that young of an age. Make sure you don't do or give anything to him before everyone else does it. For example: walking through doors, eating, greeting everyone in the home when you return, etc. If the dog is always last in line, he will learn that he is not in charge - you are. That may help in getting his aggression down.
I would suggest training classes but he's too young for them right now. But if the aggression keeps up, you and the pup would both probably benefit from them.
Talking to your vet is always the first choice though. If it's a medical problem that's bothering him, the vet can diagnose and treat it. If it's a psychological thing, the vet can also give you tips on how to help.
I hope this helps a bit... Good luck with your little one and I can't wait to see pics!
