Assalam Alykom
Its great that you removed him from any possible harm. But hitting him was not appropriate in my opinion, because since he was drunk there is other simple ways to remove him without any harm at all. But if he was not drunk i will simply call the police to deal with this matter by the time I try remove him from any harm gently. Your baby was in the car with you, I do not think your reaction was correct at all.
There are crazy people out there GP (imagine he used knive against you, and you were travelling with your baby.... Its dangerous my brother). If I am travelling with weak person such as (women/elder/baby) I would not react in violence even if that meant that someone is disrespecting me, because I would put my wife, and my baby in danger.....
May Allah reward you for your good intentions, and forgive you for your mistake amiin. In my opinion avoiding this situations would be safer.... (Unless you travelling on your own..lol)
Best regards.. Wassalam Alykom.....