How do I keep my 3 year old in a carseat?

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My almost 3 year old will not stay in his car seat when I am going down the road. I have the 5 point seat belt and he always gets out of the straps that go over his shoulders. I have tightened up the straps as far as i can and still get him in it. Do you have any ideas that I can try. I am really afraid that if I get in an accident he will get hurt. Or that he will figure out how to get out completly. Any help is appreciated. Thanks!