How Do I make my Lab less aggressive?

Right now he is 4 and a half months and I'm beginning to see some aggressiveness in him. Yesterday he got loose and when I tried to put back his collar on he kept trying to bite me aggressively like if were the enemy. Today he got loose again and he tried biting my dad when he wanted to put the collar on him. We can't do anything about the collar. We're afraid he might think its okay to try to bite. We're think of returning him cause we don't want any problems if he tries to bite another person. Thing is that we got him when he was 3 months old but he shouldn't be trying to do that to its own owners.