9 week old kitten fighting or play fighting with 17 month old cat?

I brought home a 9 week old male indoor kitten from the RSPCA a few weeks ago and slowly introduced him to my 17 month old outdoor/indoor male cat. The problem is that after a few seconds of nose sniffing the younger kitten always initiates fights with my older cat. I am unsure whether or not the two end up play fighting or actually fighting. There is no hissing or growling but the fights do seem to get out of controll, I'm scared to leave them alone when they are both inside at the same time. what do you suggest i do?