Could it be it's because they are two males? It might have been alright while the younger male was a kitten pehaps the older male is becoming aware that he could be a contender. It is probably something that they will sort out for themselves. I had two females Misty the older cat spat hissed and jumped up and down at first when introduced to Misha but as time passed (short space of time) they became fond of each other not exactly hugging buddies but very protective of each other.