Like the others have said your kitten really does need to see a vet ASAP. There is obviously something wrong there and he could be in pain etc.

It's not normal for a cat to be "flipping" when they walk, so he needs vet treatment.

When you take on a pet, you really should consider your money situation for future vet bills before you get your new pet... (I know circumstances can change while owning a pet, but your kitten is only 8 weeks old).

Like tinamary said, there are places you can go where you can get free vet treatment or help towards the costs if you are on a low income or on some kind of benefit.

PS: I just re read your post and you say your kitten is 8 weeks possible younger and 2 weeks ago he was stepped on, so you must have taken him on very young. Must have been taken away from mum too early imo).