Hi Amanda and welcome to the forum. it sounds like Cody is going to need a lot of love and patience to start showing his playful side.
I've fostered several dogs and they all come around after awhile.
It will be a lot of work and he's going to need some time to figure things out but within a few weeks you should start seeing him blossom.
Taking to the Vet and having him checked out thoroughly is a great 1st step.
I would treat him as you would any new puppy.
Start out by confining him to a small space and take him potty every hour until he gets on a schedule.
When he pees outside praise him and give him a treat...make sure to make a big deal of it when he does the right thing
If he has an accident and you catch him while going pee/poo make a low Uhhhh!!! sound and bring him outside.
If you don't catch him but find an accident clean the area really well with an enzyme cleaner like natures miracle or simple solution.
If you're sure that he's gone potty and is "empty" you cna let him out to play and eat for 20-30 minutes and then place him back in his crate or x-pen until the next potty break.
Stairs or fear over new things is as simple as providing LOTS of treats and not forcing the issue. I would bring him to the bottom of the steps and give him a small treat...next move back a little so that he has to reach for the next treat...keep inching back until he refuses to take any more treats or come up the steps and tey again later.
He'll make progress if you're patient.