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I agree with you - Antigua is unlikely (it was completely destroyed on 15th). Since Dominica was hit on 19th it provides us most probable place. But these days if you know a person’s name you can easily locate him/her via internet. Don’t you guys have Denny’s full name?

St Vincent and Grenadines and Barbados were not hit as badly as other 2 (Dominica and Antigua), however we can’t eliminate these, so total there are 4 places. There is one factor towards Dominica - it was hit on the 19th when Denny was last seen online. Antigua was almost completely wiped out on 15th.

My 2c. The first article gives us a hint - 40% taxes. Following are the countries with close to 40% taxes:
Dominica - 37.1, St. Vincent & Grenadines - 38.7, Barbados - 40.8, Antigua and Barbuda - 41%. I removed Haiti with 40.4% for obvious reasons, and US territories (e.g. St Johns). Other islands have very low rates, including the BVI (aka tax heaven). Most damaged territories are Dominica (by Maria) and Antigua and Barbuda (90% structures damaged by Irma). I think he was on one of these 2 islands.. If someone got Denny’s real name you can try finding him in local yellow pages.