Do you have pics of when first putting baby in car seat, then once slumped? I can help see if it's a fit issue then. What seat do you have?
When this happens to me it’s usually a fit issue - e.g. the head rest needs raising, or his clothes are getting in the way of him sitting back in the seat properly (like if he’s wearing a hoodie and the hood is bunched behind his neck etc) x
If the fit suggestions don't help, a last resort is to use the car seat with the seatbelt instead of the isofix base as you can usually get a better angle on the whole seat that way. It's just as safe as long as you follow the instructions properly.