There aren't a whole lot of "alligator experts" here...but if you're planning for the future when he grows, you need to be thinking more toward a significant outdoor enclosure, rather than what light bulbs you might need. In the meantime, I would suggest you contact Kevin McCurley, at NERD. He's got a fabulous, mature alligator that he keeps outdoor during the warm months, and indoor during the harsh New England winters. He could probably give you a good idea of what kind of set-up is required to keep a grown 'gator.
Good luck with it! When raised right, they are amazing creatures to have!