This is from the Celia Hammond Animal Trust website: ""When introducing a new cat do not expect them to be best friends overnight, the settling in process can easily take up to two weeks or longer. Your original cat will most likely, hiss, spit and grumble at the newcomer, this is normal and the two cats will need to sort out some initial house rules between themselves."
In most cases time is all that is needed. I would also recommend Feliway.
i dont know if feliway works ive never had to use it am lucky all my lot get on, some people swear by it and say its fantastic and others have said thier cats took no notice of it, all you can do is try it, i had similar to you only the newcomer was a right grumpy old 16 yr old due to a death in the family and a year later he still hissed and grumped at my lot but my lot ignored him ,he dosent live with me now but that isnt cos he was unfriendly he lives with my ex husband who was his connection to start with when he left the cat left
hopefully he will chill when he gets used to your resident cat it has only been a few days , feeding them at opposite ends of the same room together will reinforce that they will both get treated the same, short spells of been in the same room ensuring that your resident boy knows he is safe, maybe with two adults in the room so one of you can distract the angry newcomer and one of you fuss the resi boy, swop thier litter trays around so that each cat gets used to the smell of the other one, try feliway plug ins where the new cat is
I'm sure this has been done a million times but I have searched so I'm sorry.
Our cat is 3, we had two of them but lost his sister when she was a kitten.
On Friday we unexpectedly adopted another cat, another neutered male, but I think about 8-9 months old.
Original cat is fine, he just looks at cat 2 with lofty disinterest. However, new cat hisses and spits and has tried to attack cat 1, to the point where he frightened original cat who is normally so laid back he's horizontal.
We're now keeping them apart, new cat is absolutely fine as long as he doesn't see cat 1. He's happy to sit in places that must smell of cat 1, he just can't stand the sight of him (even through the window).