Welcome to Pet Talk Haas. I'm sorry that your first post had to be about this subject I do volunteer work for Yankee Golden Retriever Rescue and never heard of a Golden behaving this way. Unfortunately, there's always a first time. I know that in humans the anti-depression drugs take some time to have an effect, perhaps a couple of weeks. Give them a chance to work.
Have you tried to give him yummy things like cream cheese mixed with his food?
I hope your poor baby gets over whatever ails him soon. Please let us know how he's doing and give him a big hug from me.