15 gram weight loss is only about 2% of his body weight. That is easily passed in the form of urates and/or liquid. Males will frequently squirt urates and other waste all over a females tub when introduced, and that could be the reason for the weight loss. I would continue to offer what you have always fed, and he will eat again when he is ready. He may not be eating, because he is spending all of his waking time hunting for females to breed.