Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am sorry to hear that your kitten seems to have a larger than normal head which could mean that he is growing poorly, or has hydrocephalus or another sort of congenital neurologic/brain developmental issue.
Falling over could point toward poor coordination, and which could indicates a neurologic problem that can be secondary to organs that are not developing and functioning normally leading to a build up of waste products which affect brain function/coordination.
Falling over could also indicate cerbellar hypoplasia, which usually occurs due to exposure to a virus in utero which affects the part of the brain that controls coordination. If his coordination is poor he would fall frequently, have difficulty eating (which would lead to poor growth) and thus he may seem abnormally built.
There are some parasites or infections that cause brain granulomas and affect normal development.
Kittens that are growing slower than their litter mates often look like they have large heads in comparison to their body and they usually have an underlying health issue. But there can be many causes and without further diagnostics we cannot say whether this is readily treatable or not. Kittens with cerebellar hypoplasia often learn to compensate as they mature and they don't need any sort of treatment. These kittens may always have poor coordination, but if they can learn to eat and are kept indoors they can lead a happy life with a normal lifespan.
Your little one needs an examination, some blood tests and stool sample checks and with those results a treatment plan can be set up.
Please let me know if you have any further questions.