N.B.
You didn't create a brat, he's just trying to tell you that he doesn't want to do what you want. Just ignore it. If he doesn't want to sit, leave him laying on the floor and walk away. This is how he learns to sit himself up and move. If he doesn't want to sit in his carseat, buckle him in anyway and ignore the screaming until it stops, then talk to him. It's not really a tantrum, he just doesn't have words yet and it's the only way he can tell you he's unhappy with what you are doing. You "create a brat" by giving in and showing him that screaming and crying will get him what he wants.