You may want to make sure he gets a diet rich in Omega-3's like fish, salmon and such. I have heard that this can help but there are not any studies that prove it. Also he can take it in a supplement if he doesn't like fish. Lots of green vegetables are good too.
I am just a firm believer in antidepressants and haven't really concentrated on natural things to help sustain chemical balance. But I would give these a try or do some research on it. Sorry I cant be of more help.