Taxis on STT do not have car seats or boosters, though legally they have to be used for children under age 5. Sounds like your guy can legally get away with a seat belt. Everyone must wear a seat belt. As far as I know, the laws are similar in the BVIs. When we had kids that were car seat aged we brought our own. Friends who got car seats from the rental agency got car seats that were quite dirty and needed to be cleaned. Anegada may be another matter. In any case, call ahead.

Cheers, RickG