Good instincts! Essentially just wanted to echo what others have said.
Find a reputable agent. They will only get paid when you get paid. Most do not advertise unless they need someone for a very specific role, so just check the phone book and call around before you bring in your son. They will want to meet with your son, decide if they want to represent him and then will tell you what to expect and what you need. They will not try to sell you classes. If they recommend classes from another source, that's a different situation. Usually the best are through a community theater for not too much anyway. You may also be able to find some resources online or at your library on audition tips, etc for your son so he knows what to expect.
Get good, but not terribly expensive photos. You will undoubtedly get recommendations for who is "best" at headshots, but if you look around you can find starting photographers who will do a session for about $100 and give you a cd of photos that you can then print as you need. Just be sure to get examples of what type of photos the agency prefers (B&W or color, headshot only, etc.) Do not order up too many prints at once as you have no idea how many you'll go through or how quickly at first.
Work in the Midwest can be good, but often is quite sporadic. It's also important to emphasize to your son once he starts the audition process, that many times getting chosen for a role has less to do with talent and more to do with a certain "look" or other whim of the casting agent. The rejection can be very hard for kids, so keep an eye on that.
The most important thing is that he be very patient, follow directions well, and have the right attitude/personality to succeed in the business. I have acted and modeled myself but have not pursued it for my son despite urging from others, as I don't feel he has the right personality for it. I know he would have a hard time sitting still and following direction (but he is not quite 4). There are also long periods of waiting, whether at an audition or on a set (always bring a book or video game!). Then when it's your turn you need to be ready to go on a moment's notice.
If he just likes the idea of acting, not necessarily the tv or money part, then definitely check out some of the community theater options. It's a good way to get experience with reading, memorizing lines, performance, etc in a much lower pressure situation. Many theaters have summer programs or a children's performance that will have many roles for kids. Usually you can get on email lists for casting notices.
From your end, be prepared to sacrifice a lot of time and be willing to work around his schedule if he does get jobs. It could mean late nights or missing school, so decide ahead of time what you want your policy to be about those things. Minors usually need to have a parent/guardian on set, so it's not an activity like soccer where you can just drop off and pick up.
Best of luck to you and your son!