Just a thought..you left him for 10 days and it stressed him out. I had a dog I boarded once and he growled at me when I went to get him and it took a little bit for him to get back to normal, I have never left my dogs behind again. You need to spend time with him, reassure him, and build up the confidence. He probably felt confused and abandoned when you left him. He wasn't with the family he loves, I don't care if he knows the friend, it's not his family.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD