Hi there, you've gotten a ton of good advice already. When I was potty training, I had several pads around the house so I could put him/her on one as soon as I saw signs of pottying. then I would say "do your stuff" then I would treat. As they got better I eliminated a couple of the pads--
slowly.
sounds like you two are on your way to bonding. Do you think that when you pick him up from your friends home that he may be thinking you are taking him from his home and sees you as a "bad person"? When you pick him up, do you play for a few moments or do you just pick him up and leave with him? I think it might start from that point and how he perceives who you are in the scheme of things in his little world. He may just not understand where his home truly is.
I would say make sure that it is a good/fun goodbye when he leaves the other houses and goes with you. If he is not happy to leave, this will probably carry over when he's home with you. You have said that he is afraid of you and hides in his kennel. Their kennels are usually their "safe" place. And the submissive peeing is another indication of fear.
When we first got ours, I would lay on the floor and let them come over and sniff me and get to know me. then I would slowly get up, for example, on my arms, then elbows, etc. while they came over to get to know me.
Just let him figure his boundaries on his own terms and I am sure that you two will be best buddies real soon. Good luck.