has two meanings. Hissing without flattening of the ears, is not aggressive.
This is a fear response and often means "leave me alone..I don't understand you".

When the ears go down...that IS aggressive, so watch out.
Most adult cats will accept babies well. Sounds like you can let them out now
to mix. Your kittens are really small, and I don't think he would do anything to
them. (most cats will even let the kittens eat food FIRST at feeding time.)
24 hrs which you did already should be enough.

What are the sexes of the kittens? Females introduced to males are much easier than males introduced to males. Neutered toms can begin to spray if they
think their territory is being broken, but kittens don't have much of an odor to
set this off. Not all neutered males spray in retaliation. But your adult cat may do a thing I call "revenge eat"... so make sure he does not get fat as a result.
Give the kittens their own food, in a separate part of the feeding area, and give your older cat extra goodies so he will be less likely to "revenge eat".

I forgot to add... when you introduce them face to face, sit on the floor with them. You are Alpha, and your adult cat will defer to YOU and not do anything to the newbies...it works really well. Don't force them into any confrontation, but just sit there and be there.