You got some great responsesand I don't want to repeat anything, you are not a bad mommy to discipline! Discipline is a GOOD thing. I have an "all boy" as well who is 29 months old. I agree no more cup in the car, that sounds terribly dangerous. One thing that has always worked for me is teaching them to set the cup on a table when they were done or as others suggested only getting the cup at meal times/ or in the kitchen. If he is throwing other things as well, the toy that gets thrown in my house gets a time out as well, only toys are gone all day or longer once thrown.
Good luck, thankfully these are the kinds of things they do eventually outgrow, but remain consistant!