Is he in a room in his crate with the other dogs? Maybe he would do better if he doesn't feel alone? I'm just guessing here, I'm not a puppy person, but that's what came to mind when I read your post.
I agree with the special crate treats, those things worked really well with Tipper (he was 2 yrs old at the time). I put an extra special morsel in his crate and encouraged him to find it in there, that way he learned that the crate was a place to go in and find yummy stuff. I didn't shut the door at first, just let him find the treat, then after a couple times of leaving the door open, then I would shut it briefly when he went in to find his treat, and then gradually increased the time that I closed the door.