Vet check is definitely a priority .. if he's healthy ... consider what your 4-month old puppy just went through at the boarding place. I can guarantee that he had to pee / poop in his kennel / space where ever he was during the day / night because there is no way they have time to take a 4-month old puppy out every 4-5 hours.
He is likely somewhat confused ... all the rules you taught him about housebreaking went right out the window!
So, again, if he's perfectly healthy, I'd start from scratch with the potty training again!
You mentioned that he would pee in his crate - is it a big crate? If yes, make it smaller so that he's not sitting in his pee.
Marion’s Zoo-Kyleigh, Raylan-cat, Echo-TAG,