The older cat needs some time to adjust to a new comer invading its territory.
I say keep them apart in different rooms and keep an eye on them at all time.
Eventually they will get used to each others scent.
I had this same problem and with in 2 weeks they were best buddy's
If this problem keeps on going you can also use
feliway spray .
This spray is to calm cats down and its also a positive scent that cats react to telling them its safe and friendly.
Best of luck and please give the kitten a chance and don't let him go.