Hello and welcome to TCS. My first suggestion is to get another kitten, slightly older. Then they will have each other to play with.
However you will still need to play with them.

Whether you get another cat or not your kitten needs a lot, a LOT, of attention and interactive play time at this age.
My kitten, Queen Eva is just over four months old. She has three older doting cats who play with her quite a bit, but she still needs at least two good half hour sessions of interactive play with me.
I play with her in the morning for half an hour, and in the evening for half an hour, then the other cats take over and play with her, too. And she sleeps through the night, curled up against my legs under the covers usually.
So, what kind of Games do you play with Greebo? What kind of interactive toys does he (she?) have? Does your kitten have a cat tree?
Is there a window you can put a bird feeder outside of, for Cat TV?