The consensus is that true Volitan lions max out at about 15-16." The reason why I say "true" volitan is because it is not uncommon for LFSs to sell Russells lions (which are smaller) as volitan lions. So I guess my first tip would be to make absolutely sure that you have a genuine volitan.
Quite frankly, I would get a larger tank if possible. Some experts claim that you can keep a volitan in a 55 because of their lazy nature and slow swimming style, but I (and many other lionfish lovers) disagree. The biggest volitan I have ever seen was about a foot long, and even with another possible 1/2 foot left to grow, he was still far to big for a 55. Another poster on this board started a thread about a volitan that he/she saw in a LFS that was the size of a basketball; hopefully that can give you some idea as to what you might eventually be faced with, should you keep your lion into adulthood.
There are several dwarf species of lionfish that might be more aptly suited for your tank than a volitan. If you do keep the one you already have, it might be wise to feed him sparingly to avoid having him grow too quickly.
I hope this helps, and I hope some of the lion experts on this board write in, because they definitely know what they are talking about.
Good luck