Depends really on how much you want to spend. The basic ranges from Islay (10-15 year old) will cost up to £50 and if you want to spend more then you can go for an older or limited edition.
The big peaty ones generally available are: Lagavulin, Laphroaig and Ardbeg
A really nice present would be one of Port Ellen xth Releases, its a closed distillery no longer producing. Somewhat collectable and very drinkable.
Or look out for a collection of minis from a variety of distilleries.
A great deal depends, as Matt pointed out, on your budget. If you know what he generally likes (specific Islay distillery?) then you might consider buying one of the more special editions of that distillery. These can cost around 200 EUR / 300 USD / 125 GBP. A Port Ellen, like Matt suggested, will cost about the same.
There are some great gift sets that will cost around 25% of that of various distilleries. Bowmore, Ardbeg, Laphroaig come to mind. These will consist of three bottles of 20cl. This is a great way to taste some different whiskies of a less familiar distillery.
Some other (not from Islay) whiskies that are favorites of some Islay fans as well (as far as I can tell, but don't take my word for it. There should be plenty forum users who can help you out on this) are Talisker and Springbank.
As a last suggestion, there is something called the "Distiller's Edition", a special edition of 7 different whiskies, that are all considered really good. This is a sort of special edition of regular whiskies, and as such are not too expensive.
I hope you can find something nice for your son, but I am sure he will appreciate it!
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