If the clothes are clean, and fit him, it's not a worry to me. I have been a parent with virtually no money to spend on anything but the essentials, so as long as the child is cared for, I have some extra empathy in that regard.
As to the car seat, I wouldn't fret too much. Provided that the carseat generally fits him when it comes to size, of course. Is his head in a safe position? Can the straps be properly tightened around him?
It's true that you would get the ticket (I've been there!!), but doubtful that they are going to weigh the child at the side of the road. So if the seat is properly secured in your vehicle, and he is properly secured IN the seat, you are okay.