Bark in his face. I'm serious.
If that doesn't work then back tie him to a tree and tease him with a toy.
Of course when he barks you need to click/mark the behavior.
If that doesn't work then return your dog for being defective.
hahaha I've tried the whole tie him to a tree and excite him, he gets excited then just takes a seat and watches me act a fool trying to make him bark.