Aggressive/Possessive Attacking

I'm not surprised that you ended up with an aggressive dog, considering his breed, and that he was hit. 
(Groomer joke - Question: "What do you call a Shih Tzu that bites?
Answer: A Lhasa.)
Aggression against dogs has ruined many dog/human relationships, and it seems to have ruined this one - now you have a dangerous dog on your hands with a multiple bite history.  That means you, as the owner, have a dog that could be a serious liability issue, since you know he is aggressive.  So, at the least, please don't place this dog in the company of children.  What happens if someone's kid drops a piece of candy and dives to catch it at the same time your little moppet decides it belongs to him????
This is a situation that needs professional help, and not from some choke & jerk trainer that tells you that you need to be "dominant" or "alpha".  That kind of thinking makes the problem worse.  What you do need to be is the dog's "leader", and that is seldom accomplished with force.  As someone further up the thread mentioned, it is the subordinate who goes belly up, not the leader who alpha rolls the subordinate. 
 
 
 

do NOT alpha roll a Lhasa - they come back at you!  You will end up getting bitten.
Keep the leash on and in that way you can keep him from biting you and do see a behaviorist - that little guy really does think he rules the roost - .  Also, if he is acting up badly, calmly walk him into a bathroom or bedroom and turn off the lights - leave him in there for a few minutes to cool down and then quietly let him out in 3-5 minutes.  Don't say anything.  I have found Lhasas to be highly reactive and the calmer you are the better.  Slight leash corrections and either Cesar's "SSSSS" or
another word meaning stop it, should suffice.  Do not get into confrontations with him - you won't win and it will be incredibly ugly.  Stay in control - lots of exercise and not too much affection until he is behaving better.
 
Diane Gilby
Instructor