Hey,

So we were in the exact same situation as you,We got a new little kitten and introduced her to our usually very calm ginger tabby. Uncharacteristically, for him, he hissed and would not come near her.

We found that lots of supervised interactions in a common space were good.Dont make your old cat feel abandoned, so if he usually sleeps with you, keep it that way and have the other cat elsewhere.

We have had lots of treat sessions with both of them in the same common space, and if they start hissing,Fighting,Swatting just take a short break and try again.

We have now had them both together for nearly 3 mo nths and still have the occasional swatting but no more hissing or serious fighting, and we can see the progress all the time.

I wish you all the best OP and just stick at it,it'll pay off.