My Male 9 m/o kitten is trying to...?

To temporarily stop the behavior, you can try spraying your cat with water when he does it. A nine month old cat isn't a kitten anymore. Cats can reproduce at the age of 3 to 4 months. You can take your cat to get fixed now if the only thing stopping you was the thought that your cat was too young. You should make sure the mother is spade too. Thousands of cats are put to sleep every year.

Please get one or the other fixed as soon as possible. The only other solution is to keep them in separate rooms so they never have physical contact. If you don't have the money to fix them both, please at least get the mom spayed. The last thing we need is another litter of kittens, especially inbred ones. There are just too many euthanized every year because there aren't enough homes. If you're really in a bind, call your nearest humane society or SPCA and see if they have any programs for low cost spay/neuter.

Are you sure they are trying to have sex? If so just pull them apart when he tries to do it. He will learn that is a bad thing to do also you could try a squirt bottle. Consistency is key for teaching new habits and breaking bad ones.

Separate them until you can get them spayed and neutered.