I would say start with something relatively cheap, try it out, and then he will see what features he really wants. IE is a 7" screen OK to watch videos on, or will he think it's too small. Is a camera important, how much will he use the keyboard, or want HDMI out, etc. If you buy Android apps, you'll generally be able to use them on any later devices you buy. And yea, almost all of them should have Bluetooth and be able to use a Bluetooth keyboard, doublecheck each model to make sure and to find the ones that work the best with it.
The oldest tablet I have is the one I still carry the most, a Kindle Fire 7" 8G. If I were gonna get one now, I'd probably go for the Kindle Fire HD 7" 16G or even 32G, 'cause 8G is just too little. $130 (I think that's a sale price?) + $25 for the fast charger. Then get the Amazon Prime trial, and if he likes the movies and TV on there (and the free Amazon shipping) it's $99 a year. The big limitation is that it's a custom Android OS, and you're limited to getting stuff from the Kindle store which has a smaller selection than Google's Android Store. There *are* ways to "sideload" apps, and you can jailbreak it and put regular Android on there and be in the regular Android universe, but then it's a LOT harder to watch the Amazon Prime stuff - otoh there's always Netflix or Hulu+. I'm pretty sure it can use a Bluetooth keyboard, but they're all bigger than the 7" Kindle. There is a 8.9" Fire HD that would fit well with a 9" bluetooth keyboard and case, if that size works for him, but more $$$. I read on this a lot, there's a *lot* of free books from Amazon although a lot of the free ones are kind of crappy. Oh, if you don't buy the Kindle from Amazon, or buy a refurb, be careful because Amazon changes the models but keep the same name - the 2012/2013 "Kindle Fire HD" has very different features from the 2014 one.
For a "regular" 7", also consider the Nexus 7.
I recently got a refurb 9" Nook HD+ and it's pretty nice too, but JUST a little too big for me to carry all the time. This is a 2013 model but still a screaming deal. It has a really high-res display (1920x1280 pixels) and access to the Nook Store AND the Google Android Store for apps/books/video/etc. I got a combo case+bluetooth keyboard for it that's adequate, but makes it kinda fat (this one $35
http://www.amazon.com/gp/product/B00AV1UWWY). I'm thinking of just carrying the keyboard strapped to the nook instead of the fat case, although I hardly ever use the keyboard but it would protect the screen a bit. I use this around the house a lot to watch videos (netflix mainly) and do quick voice google searches. This has a micro-SDHC card slot, so base storage isn't as important. I caught a good sale to get the refurb for $100, but I just saw some on Amazon for about $130.
Last year I got a ASUS TF-201 Transformer Prime 10.1". This has a GREAT form factor, but was a little pricy with the add-on keyboard (which also has a battery which is nice, but a bit heavy). But it has AWFUL wi-fi reception, and the one I got had a problem with the charger cable that repeatedly popped up "I'm charging" "You're using the battery" icons. I didn't decide it was driving me nuts until after the return period
I think the next model has a plastic case and better wifi. I got a fair amount of use out of it but mainly it was too big to carry and annoying. I rarely use it after getting the Nook. A refurb was like $300 with the keyboard.
There are a bunch of other good 9-10", Galaxy Tab, Kindle Fire HD/HDX, etc.
I'm kind of eyeballing a Lenovo Thinkpad 10 - just came out, 10", Windows 8, 2G of ram, 64G storage, but it's like $600 and another $120 for the keyboard :p I'll wait a few months and hope there's a massive Black Friday sale. There's also an 11" Dell Venue or the Microsoft Surface, but they're pretty pricey too. I'll probably wait and see what's out next year, I'm not *dying* for a 10" Windows tablet.
OH yeah, it's worth thinking about iPads, they have a TON of apps, but the tablets are kind of pricy for the specs.
My problem is the 7" are a little too small to do spreadsheets/office stuff, or seriously watch movies, but the 9" are too big to carry handily. I have a SCOTTeVest (vest with pocket for a laptop) but I don't want to wear it *all* the time, especially in summer. I'm looking into alternatives - a holster, or some form of thin fanny pack, or some guys say a very slim case with a neck strap works for them.
Anyway, read reviews of the ones you might be interested in, compare features, keep your eye out for sales. The google search to get going is variations of "best tablets 2014", that'll get you comparison articles, then look for more reviews of the models you're considering. Good luck