Stressed cat

Hello,

I two cats - girl: bengal cross with russian blue 3 years old and boy 2years old domestic cat. A month ago I moved in with my partner for the first time to a new place and the boy started to behave out of character. He looks very stressed and every time sees my partner hides under the bed or gets as close to the floor as possible and tries to run away as fast as he can. He usualy is very laid back and friendly, doesn't mind anyone around but my partner. Now he is always under the bed and only leaves when my partner goes out or goes to sleep. He also barely eats or drinks and started urinating on everything that is my partners and in his reach. I have feliway plug in in my living room and bedroom (I live in a 1 bed flat) and catnip toys around him. My partner keeps trying to give him treats so the cat would associate him with good experience, but as soon as the cat finishes the treats he hides again. My other cat is not bullying him and just minds her own business. Does anyone have any tips or advice on how could I reduce the stress of my cat? What is there to help him cope with new arrangement and how could I make him more comfortable and used to my partner being around?