"you're golden" = "you'll be good as gold". You'll be considered very valuable.

it means "say that to him and it will work out well for you" or something like that

It means if you say that, you will turn to gold? :P Ok, maybe not. It means if you say that you will get positive results. You will be able to convey what you wanted to express and perhaps, be able to get the response you'd like.

It means he will value what you tell him.