My answer to poll is none of the above, we'll get to that in a minute.
Seeing that my wife has one and I get to work on it, your thoughts on the Spitfire are in line with mine. I don't see it serving him as a driver for the balance of high school and then college. I like the Spitfire, but it is a relatively fragile car in my mind. With a TR250 and a TR6 myself, I would still be hesitant on the TR6 thoughts. Given the budget mentioned and the nominal prices of the TR6 now, I don't see that working for you unless you really luck into a good deal. And then we get into the maintaining of things again. The wedge cars have their own issues and parts support doesn't seem to be as good for them as it is for the earlier TR series cars. Besides, all of these are climbing on the collectibility ladder which in and of itself can present issues when one is put in use as a daily driver, especially if ungaraged as it would be at your typical college parking facility. And this is where ask the insurance company advice can really come in handy. A car that's used as a daily driver or an ungaraged car can't go legitimately on a collector policy and your typical daily driver insurance policy is just going to see a Triumph as an old crock that isn't worth any money. If you are lucky, they might allow a declared value policy for a daily driver but if so, I would expect it to come at a price.
(Looks like the insurance aspect was addressed while writing this post)
My suggestion, if you want to stay within the 2 seat sports car realm, would be what I've jokingly heard is the automotive answer to everything - a Miata. I realize that this can be viewed as a bit of sacrilege on the part of a many times over British car owner, but hear me out on this. For the kind of budget you're talking about, you can get a rather nice NB series (1998 - 2005). You can also look at an NA series but now we're talking of cars that are rapidly approaching 20 years old if they haven't gotten there already and I don't care how relatively bullet proof something is, after a while stuff is going to break. Another factor with the NA and especially the earliest ones is that they too are starting to climb the collectibility ladder so it getting hard to get a really good deal on one. There is also the possibility that with the introduction of the ND series which go on sale next spring, that early NC models may be in your budget range shortly, although I continue to think that the NB is your best bet. It's somewhat more refined than an NA, but is still raw enough to have a somewhat Spartan feel like that of the Triumphs, but with air conditioning, nice to have in the Tampa area on occasion. The NC is a "better car" than the NA or NB, but I'm not so sure that I would say it's a "better sports car." The NB is not as complicated as the NC, has that bullet proof Mazda twin cam engine that can be built to take well in excess of 250 HP as opposed to the MZR engine they share with Ford (not a bad engine but not as robust as the old Mazda twin cam), good parts supplies and lots of aftermarket support, insurance is reasonable (relatively speaking, a 16 year olds insurance rates are rather scary no matter what it seems), etc.
If you've never given them much thought, here's a link to a pretty decent Miata site to check out:
https://miatanet.com/
Turns out that there are a large number of British cars owned by users of that site, I bet that there's a fair number of users of this site that have a Miata (or several) stashed in their garage keeping the British cars company. In the interest of full disclosure there is an NC with a retractable hardtop sharing space with the Triumphs in our garage.