crate training seems like it’s going to be a nightmare

Hi everyone! My girlfriend recently got a 4 month old cavalier king charles spaniel. We’ve only had him for 3 days so everyone is very much still settling in and learning the new routines. He also was living with his litter mates and mom until a few days ago so of course it makes sense that he’s clingy and wants to be held 24/7. He’s mostly fine in the playpen if someone is nearby but about 15 seconds into being alone the barking and whining starts.

I guess I’m mostly wondering how I should set everyone up for success, especially with crate training. The longest he’ll need to be alone for eventually is 3 hours but right now that seems like an eternity to build up to. I’m worried about contributing to his separation anxiety and don’t know how to make his crate/playpen a fun and safe space.

Unfortunately we haven’t yet found any treat that he loves, he doesn’t seem to be very food motivated at all (maybe he’s just in shock though and that will change)?? So many strategies around crate training involve treats, lick mats, feeding their meals in crates, and all of those seem to be so ineffective for him at this point.

Apologies for seeming so stressed so early on- I know it’s a big adjustment for all of us. I definitely have some puppy blues but hopefully with time it will get easier.