I have a 7 month old cat & I just recently got a two month old kitten..

My 7 month old cat is aggressive towards my kitten. I’ve introduced them & put them in separate rooms so the can interact through under the door, but my 7 month old cat is still so mean.. Anytime they are in the same room my older cat will chase my kitten down & bite or hit him.. how do I get them to be nice?

Also, my kitten is having a hard time using the liter box.. he will whine & we will take him to the liter box but jumps out, so we have to put him in there about 20 times before he actually uses it.. Half the time he’ll run to the corner of my bedroom & do his business there or go right beside the liter box.. the liter is always clean, so I’m not sure why he’s doing so.. help!