if you are adamant that you have put him in correctly then get rid of the seat and buy a new one.
Make sure he cant get out of it, there are clips that can be bought that hold the 2 sides together making it more difficult for baby to pull them apartbut there are doubts as to wether these are safe should baby need to be rescued from a car seat.
Still if hes out then anythings safer.
As soon as he starts to wriggle stop your car at the earliest safe opportunity. Do this for now, until you can get a new seat.
I use the Maxi-cosi XP which i find great, my son can get out of highchair straps but not these.
http://www.halfords.com/webapp/wcs/stores/serv let/product_storeId_10001_catalogId_10151_prod uctId_169693_langId_-1_CarSelectorCatalogId__C arSelectorGroupId__varient__categoryId_89376_c rumb_33949_parentcategoryrn_89376 it is possible to get these cheaper but as i said, get the seat checked in your car at halfords or mothercare