I would suggest asking your vet about a possible inner ear infection impacting his balance center. That can make a cat 'dizzy' so it can't walk well.
It is also possible you are dealing with a problem that is neurological with something impacting the brain and perhaps the cerebellum which controls motion.
Parasite migrations and infections such as roundworms, Cuterebra larvae, toxoplasmosis can cause neurologic signs in kittens as the parasites can impact the brain if they get 'lost'.
Portosystemic or liver shunts can cause neurological problems too.
You may need to do some blood work and more testing or even get a referral to a veterinary neurologist to figure out what is going on with this little one.
Hope this helps you!