I've trained two dogs now, a pitbull and a goldendoodle, without using domination techniques. My dogs are my friends and family. Specifically, I followed the Zak George school of training. I'd invite you to consider his philosophy and consider his insights here for puppy biting: https://www.youtube.com/watch?v=JRl1FhIBeKc
I am nowhere near experienced enough to extrapolate what the dog is feeling or what happened to him while being boarded, but the most effective way I've found to train my dogs is to find their favorite "currency." Does your dog enjoy treats ("food currency") or playing ("play currency")? (My pitbull is food, my goldendoodle is play!)
I believe it may have also been Zak who described the dog's mouth as his "hand;" the dog's teeth are very sensitive to pressure and texture. They know the texture of human skin.
When my doodle was a puppy (though I am sure this will work at any age), if he got too excited and bit my skin, I would let out a loud "OUCH" and play would immediately stop, restarting a short ten seconds later. After reinforcing this behavior over several weeks, my doodle was very careful with playing and the puppy biting stopped. The "OUCH" technique then transitioned to stop the doodle from biting at my fiance's clothing. If he leaped and tried to bit at her sleeves, she would exclaim "OUCH" and the doodle understood it as a cue to stop.
I transitioned from "OUCH" into "NO" in his training. Those are in CAPS as instead of using a high pitched praising voice, it is a more firm lower pitched tone.
I would invite the OP to please consider not using dominating techniques; as documented here:
Explains how the scarcity of dog information before the advent of the internet helped perpetuate the dominate training myth. Explains that Dogs are not Wolves. Begins to explain that biting and bad behavior; has nothing to do with dominance. Explains how biting is attributed to fear! Argues that there is no scientific evidence to advocate the actual existence of "dominance" in a domesticated animals.
The trainer's most important video posted. Explains that dogs are significantly different from wolves. Goes into deep detail regarding the "dog jumping on you" as form of dominance. Incorrect, according to the author. Dog jumping on you is a learned behavior, and excitement. Use positive reinforcement to perform proper training.
https://www.youtube.com/watch?v=kcGvbVTN4yk (specifically this one)
Covers the debunking of the debunking of positive reinforcement. Covers the reason why the myth of dominance is perpetuated.