I don't know where the 2nd slot fits on your child, or if he is rear facing or forward facing. So first, make sure that that the straps are coming out at or below his shoulders if he's rear facing, or at or above his shoulders if he's forward facing. Also, make sure the crotch buckle is coming out in the correct spot between his legs and isn't too far forward or back. When it's tightened, can you fit 2 fingers between the straps and his collarbone, or pinch any slack? If so it's too loose.
If it is indeed properly tightened and he is pushing the chest clip down to wiggle out, there are a few things you can try. Do you have the HUGS (the rubber grip things) on the straps? Those may help keep him from wiggling out. Also, you can put some velcro on the chest clip, just the prickly side, so it's not as pleasant to touch. You can put him in a hoodie, and zip the hoodie over the harness. If he can unzip, try a sweater that snaps or buttons.