Your cat obviously feels under threat from the new 'pack member' and feels his territory is no longer his own.
The suggestion by sammy007 is a good one - let your cat see that the kitten is actually no real threat - without the facility for him to attack the little one - and hopefully he will assimilate the kitten into his life, and home.
When the kitten grows to full size, there will be the usual jostling for position, but at least your kitten will be able to punch his weight in a fairer fight.