I fostered some 6 week old kittens a couple of years ago and my Paddy (then 9 months old) was very sweet with them. He growled and hissed at them for a couple of days but then started to play with them ever so gently.
Here is a really short mobile phone video of them playing outside. He really missed them when they went off to their new home.
My other cat at the time, Maddy (same age as Pads) was less impressed and even after 2 weeks, wouldn't come near the little ones but would watch them from a distance from a high place without growling until they came near her.
I think that different cats just take different amounts of time to adjust to a new kitten and as long as they are supervised for a while yet, I'm sure they will be great friends.