You should use a PWM Controller. They will not drain your battery or get red hot, like a headlight dimmer would. I use this on my 20 million spot light. I will be adding one to my headlight soon.
A dimmer is used to dim the light so its not so powerful. You can use a dimmer so you will not have to lift up the light above his eyes. So the closer you get the dimmer you can make it. You usually just want to be able to see the glow from the alligators eyes.
You can easily buy one from here.