He's not running on empty though, he's calorie cutting, which is a technique commonly used for losing weight quickly. Although, it is quite extreme you have dropped down to 500, I think the lowest I've heard was 1-1200, perhaps you were hoping for quicker results?
But since you have reached your goal weight, you should not attempt to go further, as that would suggest you did have an eating disorder, and you would then become anorexic. Focus on eating more calories per day, even if it's 100 extra per couple of days, you'll soon be back up to 1,000 then remain stagnant for a while and improve again slowly, not too quick. Make sure you keep exercising though so the weight you're putting on is muscle mass and not just putting you back at the weight you were before.