You can ask God Gale for the sunlight cure, not a vampirism cure. There is no dialogue about curing his vampirism at all.
I apologise I specifically mean a cure to the sunlight not necessarily his vampirism. I'm thinking more along the enchanted armor or spell path hypothetically Tav is his one and only most likely will continue feeding him and on their enemies if Tav is a High Elf they could be together for a further couple of centuries when Tav's time is up and has not tried Jaheiras Immortality ritual Astarion can remove the enchantment and walk into the sun.
