B.C.
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!
Good luck
B.