We really need advice!... Our Cat's are fighting!

Sylvester has been with us for a week.
During that week he was sick with the flu.
Now that he is better
he has begun attacking our younger female cat.

Bobbi (female) on the other hand
started hissing at him the very fisrt day we brought him home.
She shows interest in his whereabouts
but tries her best to avoid him.

Today was actually the first time he attacked her.
They were both near the kitty litter.
She has hers and he has his.
ALSO, Bobbi has for some reason started
rolling around on the ground near his kitty litter....
And sometimes uses his instead of hers...

Unfortunately we weren't able to introduce them to each other slowly
as sylvester was sick when we brought him home
As he was very quiet, and docile the first week,
we didn't think that we would need to keep them in separate rooms etc.

What could it be?
And is there a remedy for all this fighting?
It's very important to us that they do get along
as we live in an apartment.

The attacks are getting worse!

He has been neutered.
She has yet to be spayed.
Could this be the reason why he is attacking her?

is feliway a possible remedy to this siyuation?

Sylvester is otherwise a very sweet natured mild mannered cat
and for that matter so is bobbi...

It sounds as if you introduced them much too quickly. The male has now learned that his role is to attack the female, so he will keep doing this unless you separate the two and start completely over. So, please immediately separate the cats and then follow the directions below.

Introducing a New Cat

First, put the new kitty in his own room. He will need to stay there for several weeks, so make sure it is comfortable for him with litter box, water, and food. This will allow your old cat to get used to the sounds and smells of a new cat without having her territory invaded.

After a couple of days, start trading scents between the cats. Rub the new cat (especially around the cheeks) with a slightly damp towel and then go rub the old cat with the same towel (and vice versa). This way she will associate the scent of the new cat with good things (being rubbed and getting attention). Do this several times daily.

After one week, lock up your old cat and let the new cat out for the night or for a few hours. Do not yet allow the new and old cats to be together. Do this for at least two weeks. (Most introductions do not need such a long separation, but since there has been aggression, you need the separation to last extra long so he can "forget" that he likes to attack the female.)

By the end of week three, you can try bringing the new cat out with the old cat for brief visits. As soon as the new cat comes out, give them both some tuna or some other really yummy treat. This way they associate great things with the other cat. Put the new cat back in his room after a brief period of time. Make sure you keep any aggression from occurring (use a squirt bottle or GENTLY toss a pillow at the aggressor).

Gradually increase the amount of time the cats are together. I recommend keeping new kitty in his room over night for at least a fourth week. When the cats are out together, make sure you give
lots of attention to both cats.

Expect lots of hissing and perhaps a swat here and there. But...this should keep any real aggression from occurring. The hissing will stop once they establish their hierarchy and get used to each other.

Most importantly....have lots of patience and take things sloooooowly!

What I have started doing is, using our bedroom as a neutral sort of area.

Sylvester(new cat) spends the nights in the living room as bobbi(old cat) prefers to sleep with us.
Then during the day, I let bobbi out while sylvester has a morning nap in our bedroom.
And then after lunch bobbi comes back in the bed room and sylvester goes out side again.
And then I swop them again in the evening.
And finally bobbi comes in when we go to sleep.

They are both aware of each other...

Is it advisable to continue like this?

They don't seem to mind...

The other problem is that we are going on holiday on the 19th for a week.
We don't mind leaving Bobbi here with the house sitter, but I don't think we should leave them both here as the house sitter may not be able to handle the situation.
So we may have to leave Sylvester with a friend of ours for a week,
from the way things look this might make re-introducing them when we get back a bit easier...

Any further thoughts?
Thanks