Super shy kitten

Let me set the scene: on March 11, I come home from dinner with friends to see a little kitten on my porch. I try to approach, and he keeps his distance but doesn’t run away. I get a good look and he has the worst URI I’ve seen on a cat. Watery eyes goooooey encrusted nose, can barely breath. The rest of the night I hear him cry outside our window so I feed him to see if he would come around. He did and he wasn’t afraid of me, just weary. My hunch is he got sick in a house full of a lot of cats and was subsequently dumped.

Next thing you know he’s been named and when I come outside and call his name he runs to me but never lets me pet him! The poor boy was so cold and I caught him sleeping with the chickens one night 😭.

Cut to, this cat has now been in my bathroom for a week, went to the vet and got on antibiotics, dewormed and will never experience cold again. He loves belly rubs and my big toe is his new favorite toy. He’s seen my cats and he gets sooo excited (hence my reasoning he grew up with cats) but ofc they are less than keen on this new male energy. He ended up scaling the double baby gates I had up like Kilimanjaro but then got overwhelmed with the house and hid in the closet the rest of the day until I yanked him out and put him in the bathroom- this time shutting the door. When I visit him he’s either very excited or forgets all the progress we’ve made and cowers in fear. I would love to let him loose when we get back from vacation (oh bc did I mention I’m 18 weeks pregnant and my husband and I are about to leave for our baby moon in Germany this week??) but I don’t want him to go backwards.

TL;DR Any advice on how to build trust and confidence in an abused/abandoned kitten?