I bought my 13 year old daughter a used pink Diamond Infinite Edge a couple years ago for around $225 shipped. She really likes it. The sight and rest that come on it are cheap. The rest is ok to start off but I replaced it with a whisker biscuit. I don't think many will argue that the whisker biscuit is the best rest you can put on a kids bow. I have used a biscuit on my hunting bow for the last 15 years. I would recommend a single pin sight for a kids bow. It will be a while before your son has to worry about shooting longer distances where more than one pin is needed.
I picked up a used Mission Mennace bow for my 11 year old son a couple years ago for about $160 shipped. Mission still makes this bow I believe but they changed the name of it. It is a nice bow but the draw weight doesn't adjust down as low as the Infinite Edge. My son had a hard time pulling it back for a while. I put a whisker biscuit and a single pin sight on this bow as well. Once he was big enough to pull it back he has had a ton of fun shooting it. Overall I have been happy with both of these bows but I think I would say I like the Infinite Edge a little better.
Here are a few things to keep in mind when getting your son started:
1. Figure out if he is left or right eye dominant. I'm right handed but left eye dominant. I shot right handed when I was younger but switched to shooting left handed about 20 years ago. I shoot so much better left handed. My kids both shoot left handed. My son is left eye dominant. I kind of pushed my daughter into shooting left handed just because I had an extra left handed sight and rest to put on her bow. Lol.
2. Be prepared that your son might get frustrated if he doesn't shoot well at first. Both of my kids have gotten frustrated and wanted to quit shooting. You aren't going to be shooting bullets out of a kids bow that has a low draw weight between 5-25 pounds. I gave my kids some old arrows I had that are pretty heavy and 2-3 inches too long. I realize these arrows aren't ideal for their set ups but they only shoot 10-15 yards. My kids draw lengths have changed every year sometimes more than once a year. When they quit growing I'll make them up some arrows that will better fit their draw lengths. My kids just shoot for fun, they have a long way to go before they are ready to bow hunt. Start your son out at 5 yards or so and keep it fun for him. My kids have a 2x2 foam target. They get bored shooting it after about 30 shots. Change it up, my kids have the most fun trying to shoot at a water bottle, soda can, and balloons.
3. Explain to your son how important good consistent form is to shooting a bow accurately. My kids struggle with this mostly because they are at that age where "they know everything" lol. Another thing to think about is if he will shoot fingers or with a release. My kids both shoot fingers. I know shooting fingers isn't a bad thing but I know my kids would shoot better if they tried a release.
4. Make sure your son knows that you should never draw a bow back unless there is an arrow on the string. The day I took my daughter to my local shop to get a new rest she dry fired her bow in the shop. Lol. We were lucky the draw weight was set pretty low that it didn't hurt the bow. I guess we were lucky she did it at the shop. The owner checked it over and had it fixed in less than 5 minutes.
5. Don't be afraid to ask for help. If you aren't a member on Archery Talk yet, check it out. Archery Talk is my favorite forum and has an unbelievable classified section. I have been a member there for about 10 years. I bought both of my kids bows there and I have bought 4-5 bows for myself there. I shoot left handed so it is impossible to find a good selection of left handed bows to try at local shops. The only new bow I ever bought was a new left over bow. I wouldn't hesitate to buy a used bow on Archery Talk. Check out the sellers feedback and you shouldn't have any problem. You should be able to find a used Infinite Edge there for between $225-$300 shipped and you will probably find one with an upgraded sight and rest.