Ozzie says, "Geez, dude, I'm just not into treats. Just throw the damn ball!"

Sorry, it's kind of funny from the outside that you're both probably equally frustrated. But, maybe that's a place to start, just recognizing it. There's really nothing he's consistently into?

 

I want to suggest that that your dog is not unmotivated, but that he not food-motivated. There is a difference.

It's up to you as a trainer to find the "currency" that will motivate him and find a way to use that "currency". For example, many dogs that aren't food-motivated respond well to Premack-type training.

 

You aren't alone +two. Caeda is only food motivated in the house when nothing is going on, outside she couldn't care less. There are even some days inside that she just won't "perform" for treats.
Like Poly and others say (and I'm sure you know), just find the right "currency". We've found that the absolute ultimate currency for Caeda is the laser, unfortunately it only works for certain things since it gets her so excited (thankfully it worked wonders on recall, though useless for leash walking). Sometimes, in some circumstances tug does the job, but also comes with the excess excitement factor some days.
Either way, you're not the only one. We have yet to find anything that consistently motivates Caeda and will work for most training scenarios. I really know your frustration