A baby chick that is brave enough to run up to a giant (you) is not being "sweet" it is being dominant. Your rooster learned that this is acceptable behavior and is doing the more mature version of it now. It must be confusing to him since he was rewarded for it as a baby. Kicking him will encourage him to fight harder or attack you from behind. It also causes deep bruising, internal damage and can break his bones. What it won't do is teach him to stop since roosters aren't known for their ability to reason.
How do I get him to stop then? He always seems submissive after we beat him at his own game. For the bruising and internal bleeding, I never kick him hard, just enough to push him back.