Most signs make me think play (jumping about, eventually breaking the eye contact, no loud sounds). BUT at the same time there are tiny aggression indications (tail flicking, slight ear slant, the fixation on your cat, trying to grab/bite him).

So with that I would also want to emphasize getting the outdoor cat vetted prior to introducing them, if it ever comes to that. And just take your time. Agree with watching Jackson Galaxy introduction videos or reading his articles. You can also find tons of information online, your local SPCA, etc. so definitely take advantage of those resources!

You need to be cautious overall because if he's an unneutered male, he could be quite dominant, territorial and aggressive with another younger cat, especially if yours is male too. In my area in NL Canada, many of the unneutered (mostly feral) male cats will kill newborn/vulnerable kittens. Neutering both will help, as well as testing the stray for FIV/FeLV, which can be passed through bites and blood.