Unfortunately, apple stopped making iPod touches. So, you have a few choices.
You could get an iPad mini - the most expensive option but also the option that will probably last the longest.
You can get a refurbished newer iPod touch with more storage from Apple, but keep in mind that within the next year, Apple will probably stop supporting the operating system on this.
Or, you can get a iPhone 5 or 6 and don't give him a cell plan. If you get an iPhone and simply don't attach it to a cell plan, it works as an iPod. This is what both my kids have. When my DH and I updated our phones to a newer one, we gave them the old ones to use as iPods. So - do you want an iPhone 7? You could upgrade and give your son your 6. Or, you can look on the Apple site for refurbished older iPhones or you could look on Craigslist, etc, for old iphones. Again, keep in mind that Apple will probably stop supporting the iOS for iPhone 5 next fall, so that is on borrowed time just like the 5th generation iTouch.
ETA: Sorry I was wrong about the iTouches (thanks to another poster for correcting me). I swear I thought they stopped at the 5th generation. But they do still make them, and you can get the 6th generation on the Apple refurbished site in addition to buying new the 7th generation.