I don't know of a tablet with built-in browser filtering. When they say they have parental controls, most of these devices mean they let you do things like: turn off web browsing entirely, limit hours of use per day (or hours of watching videos vs. reading), and so on. They don't come with controls that let you filter content from the internet, it's either no web browsing at all or regular unfiltered browsing. You can read a little more about that here: http://www.techhive.com/article/2015665/parental-controls-face-off.html
There are third-party apps that you can get that limit browsing, but some you have to pay for, and they all work differently. I don't really know anything about those because I don't use them. Since this is important to you, you'll want to make sure there are decent and affordable apps available for whatever device you're leaning toward. Maybe someone else will be able to chime in.
As far as the devices themselves, if his main use will be reading, light browsing of news sites and so on, videos, and occasional email, any of them will be fine.
Nook HD -- has a really nice screen, light and easy to hold, seemed to work pretty smoothly when I tried it for a few minutes. You do need to register it with bn.com and have a valid credit card on file, even if you are just downloading free books and apps. Access to Barnes and Noble store. Has the fewest apps by far, so even though it has parental controls, it may or may not have a browser filtering app.
Kindle Fire -- pretty similar to Nook HD. Your books will mostly come from Amazon. I know people who own these and are happy with them, but I've never used one beyond helping library patrons set them up to check out ebooks (which you can do on any of these tablets as long as your local library system is set up for that). I won't get one because I don't want my reader tied in with Amazon.
iPad mini -- More expensive than either, not set up to be a dedicated reader, but can get books through both Kindle app and BN.com reader. My mom has one, but she doesn't use it for reading -- mostly email and games, since she has an e-ink reader for reading. So for her it's a way to get online when their family computer is in use. As far as parental controls, apparently you can disable the Safari browser and download a filtered browser.
Various android tablets like the Google Nexus and Galaxy Tab -- Like the iPad, they're not set up to be dedicated readers, and they have access to the android app store instead of apple. They're much less locked down than either the Nook or Kindle, so that may not be what you want.