I think option one would work. I'm not sure on exactly how glamours work in Eorzea but I'd assume the horns and tail would still be there, just... not visible. But like the others above me have said, it's really up to you since it is your character.
I don't really side with the idea of ditching the character completely though, especially if you can come up with ways to make the change in a way which is fun and plausible to you and those you RP with. If you're attached to the character and have put a lot of work into him, ditching him completely might be hard (not to mention the complications caused ICly, especially if the character is in a position that affects others). And you've already said that you don't want to go down that route anyway.
I think option one would be a good one, but that's just personal opinion. I'm not a fan of the 'oops I accidentally grew horns and a tail' idea, though I have seen it done before and it was a fun little episode (it was more for silly comic relief at the time). Fantasia doesn't exist within the lore, but glamours seem to be fairly common and lore abiding. Whether the tail and horns would be gone completely or whether they'd still be there but invisible I don't know. Then again, if he's been hiding it for this long then I'm sure he's mastered the art of not knocking everything off the table with his tail every time he turns around.
That said, I think you should go for whichever one seems the most interesting to you. I think that is option 1, but like I say, that's just an opinion. Personally I like the simplicity of it and the fact that your character can control the reveal somewhat rather than it being 'Oops. Surprise!'
Edit: I may have misread your first post initially. I was assuming that he'd be aware of the glamour and possibly keeping it up himself to help protect his disguise (I don't know if there is an actual time-limit for glamours or whether once applied to a living being they would stay put forever). If he isn't aware of it and the transformation comes as a surprise to him as well as to everyone else... it could work, but like K'nahli said, surely he'd have noticed something at some point?
I don't really side with the idea of ditching the character completely though, especially if you can come up with ways to make the change in a way which is fun and plausible to you and those you RP with. If you're attached to the character and have put a lot of work into him, ditching him completely might be hard (not to mention the complications caused ICly, especially if the character is in a position that affects others). And you've already said that you don't want to go down that route anyway.
I think option one would be a good one, but that's just personal opinion. I'm not a fan of the 'oops I accidentally grew horns and a tail' idea, though I have seen it done before and it was a fun little episode (it was more for silly comic relief at the time). Fantasia doesn't exist within the lore, but glamours seem to be fairly common and lore abiding. Whether the tail and horns would be gone completely or whether they'd still be there but invisible I don't know. Then again, if he's been hiding it for this long then I'm sure he's mastered the art of not knocking everything off the table with his tail every time he turns around.
That said, I think you should go for whichever one seems the most interesting to you. I think that is option 1, but like I say, that's just an opinion. Personally I like the simplicity of it and the fact that your character can control the reveal somewhat rather than it being 'Oops. Surprise!'
Edit: I may have misread your first post initially. I was assuming that he'd be aware of the glamour and possibly keeping it up himself to help protect his disguise (I don't know if there is an actual time-limit for glamours or whether once applied to a living being they would stay put forever). If he isn't aware of it and the transformation comes as a surprise to him as well as to everyone else... it could work, but like K'nahli said, surely he'd have noticed something at some point?
