Finding a used horn at a decent price can be a challenge.
There are a number of things to consider. Your son is presumably playing a single F horn as opposed to a double. If he continues to play horn, it is likely that somewhere between 7th and 9th grade (probably at the earlier end of that range) he will want or need to move to a "double" horn. The double horn is essentially two horns in one and allows for a better high range. It is just best to learn on a single. There are also players who go through high school on a single but not very many. As you look to purchase, you should consider that the horn you are about to buy will only get a few years use assuming he continues to play.
You will find a lot of variation in the quality of used horns. What I would say is most critical is that the valves are in good working order. Valve repairs can cost more than the instrument if there is anything seriously wrong with them. Make certain there isn't much play in the valves. You can do this by turning the horn over and looking for the largest screw on the back of each valve. Grab the post that the largest screw goes into and attempt to pull it out away from the horn (pull it up and down that way to check the play in it). It should not move. If it moves too much, the valves likely have an issue with the bearings. That doesn't mean it won't play ok today, but it means that at some point it will need repairs that can range from $200 - $900. I also unscrew the caps on the top of the horn beneath each valve. You will see a scoring on the part of the valve that moves and on the rim around it. If the valves are properly lined up, those two marks should line up. Some horns have a scoring mark that lines up both when the valve is depressed and when it is not. Some only have one. Look at each valve.
I would also make certain there are not too many large dents or any worn spots (where it appears the metal is thinning out). Blemishes in the finish don't matter and you will see them in every used horn. Large dents can alter the tone and playability of the horn, but many can also be removed by a good brass repair person. Wear spots will ultimately need to be patched. It is hard enough to play the horn without having a leak in the middle of it (sort of like sucking through a straw with a hole in the middle).
Finally, I would have your son or someone in your family who can get some sound out of it play it a little. It is amazing how different even two horns in decent condition can feel. It is also a way to check that the valves are at least line up decently. There should be a nice free blowing feel and sound.
As far as brands, there are a number of decent brands for student single horns. Holton, Yamaha, Conn are probably the most popular. You can also have good luck with Bach, King, and Olds. There are not a lot of other brands that I would try. Ultimately, you just need one that he feels comfortable holding and playing. That is the most important thing. Have him try a scale or something very basic on it.
Finding a used horn at a decent price can be very difficult. You can find a new single horn from $1,200 or so and up. Sometimes you will see used ones at that $1,200 too (but I wouldn't pay that much for a used single horn). Don't assume that a new one will always work better even at a good price. I have tried new horns that I didn't like near as well as older ones just based on the way they played and sounded. It is unlikely that you will find a decent used one for much less than $600 - $700. You may get lucky, but you will want a chance to try it out.
If your rental was a used horn, you might consider buying it. I would also ask your son's band director and try other area music stores to see what used horns they have or are aware of. Music Center of Deerfield sometimes has some. Libertyville Music is also affiliated with them. They are frequently overpriced in my view, but they get some good instruments. I would look up all local music stores and ask. You can also try a website called hornplayer.net. There is a used horn listing there (go to Single F). There are not a lot of single horn options there right now, but there are generally a few. There you at least find you are getting sales by horn players with better descriptions of what you are buying than on most sites. I would look for something reasonably close to home so that you can potentially try it. There is one very old horn for sale in Chicago right now. eBay also has some listings. Again there are a lot of bad ones. Right now there are some fairly low prices from a place in Centralia, IL and one from PA. If it's cheap enough, you might decide to take your chances, but you should be prepared to find it is not a good instrument. Sometimes a place called wwbw.com (Woodwind Brasswind) has good scratch and dent sales. This is a reputable dealer in South Bend, IN and worth a look as they are honest about what they are selling. You are just likely to end up at $1,200 for a blemished newer horn if you look there.
If your son is currently renting a brand new horn, prepare him for the fact that the used ones will look pretty beat up to him. They can, however, be as good or better when you play them. Additionally, spend the extra money in a few years when he needs a double horn.
I'm sorry to ramble on so much for a seemingly simple question. If this is not clear or if more information would be helpful, please post again. I hope your son has many years of enjoyment out of playing the horn. It is a very difficult instrument to start, but it is a lot of fun and very rewarding as you progress.
