So, the new kitten, who has not been used to such a big space, has been given the run around of the house, and the old cat, has had her territory taken away from her, and has been usurped by a youngster. No wonder she is hurt, and has reacted badly.
Why could the kitten not stay in one room for the day? Why should the senior have to have any rights taken from her/him? Of course he is going to defend his territory, it seems you have set yourself up for a lot of trouble by taking away from old cat what he has had for a long time and giving it to kitten.
I understand you regret this. But, for all parties concerned, your son at least, getting rid of the cat, is setting a really bad message, and not the most responsible thing to do.
My sister recently took in her second ferral, and managed to get the two, one old, and one kitten settled within 3 weeks. The kitten as a child would have to settle for less, if the old cat were to accept the new and help "raising" him he would have to keep his rights and his territory, and slowly let some of it befall the youngster. The kitten spent a lot of time in a cage to start with. And old cat was given plenty of cuddles and praise. Now they have a blissful situation where old cat is helping look after the kitten, and will HISS to get the kitten down from tables and other places he shouldt be.
Try giving your cat his territory back. Let him roam the house. Let the kitten have a bedroom, while you are away. Let them both have the entire house while you are home. Keep an eye on him, when he enters the garden, etc. As he has to get used to that too.
Good Luck.