I don't have a child in acting, but we did work a lot with children at the ad agency both for photography and TV commercials.
You definitely want to find an agent or a company. NEVER pay upfront for anything, and you don't need to pay a lot for professional photo shoots or anything. Any company that tells you that is trying to make money off of you. A simple headshot and body shot (and even one you take yourself) is fine. You should definitely not pay to have your son featured in a book that "goes to all the companies" because that's not how it works. We contact the agency, they send us the candidates and we choose them based on their photos (again, just headshots and body shots) and recommendations.
It's an interesting business, easy to get "caught up" in, but it can be rewarding. Just be prepared for a LOT of rejection. Honestly, the way we chose children for our TV commercials was basically based on our mood of the day! We picked the kid that we could "picture" being in the commercial and that was it. So don't take rejection as being a commentary on your son and don't let him take it personally. Sometimes we want a little brown-haired boy with cute cheeks, sometimes we want a blonde girl with curls. Pretty much all the kids are "charming." I think that's the hardest part, the rejection--and it's not even personal. I could tell a lot of moms would get in snits at auditions, or try to make their child be "more" what we were looking for. Don't be that mom! Either your child has it or they don't (and they might not be right for this spot, but they could be right for the next).
Good luck!