I have a 12 year old male cat that's fixed. Last Novemeber we got a male kitten that's now almost 10 months old and not fixed. The two never really got along, but they could be in the same room without killing each other. But for the past two weeks, the kitten has been attacking the older cat. We can't even have them in the same room with each other.
Could it be because the kitten isn't fixed?

Possibly. He may be getting old enough to start exerting himself, and yes trying to claim territory. Cats can do this anyway, but a non-fixed male can be terrible about it.

He needs fixed anyway, so why not just take care of it and see if the problem takes care of itself? 10 months may seem like a "kitten" to you, but that's getting a bit late.

Please get the cat fixed. Soon he will start spraying too, a very awful-smelling urine odor. Nine-ten months is about the limits for being a "kitten."