I known this is very upsetting to you because these are your babies and you want them to get along like they were.
You do not need a behaviorist for cats. They are really quite easy to understand.
They do not rationalize, nor are they capable of in depth thought or reasoning.
Everything they do is basic, logical (to them) and makes perfect sense.
In a cat heirarchy a strong kitty will always try to take over and assume the head cat position. In addition, that is always subject to changes if cat one senses weakness in cat two. Cats DO work things out in their own way and they could care less what humans (or behaviorists) think. They only respond to the energy of another animal be it feline, canine or human. Punishment is useless and counterproductive because they simply do not understand being hurt - they would just try to avoid the hurter in the future but it would not teach them anything.(except fear)
Cats do not follow a pack leader the way dogs do but they do sense their human moms energy and respond accordingly - so if you are nervous, they will be nervous. If you are stressed, they are stressed as well.
Since they have been together a while there is no getting to know you phase. They already know what to expect from each other.
Cats do not recognize others by sight, they depend more on scent, so if some tumil is going on in or oudoors, they can sometimes take it out on each other and begin hissing and spitting.
Something spooked cat one so he blames cat two - he does not understand that an ouside influince caused the initial prolem.
I know this sounds simple - because it is - just be a calm, assertive, loving energy in their lives. When they are at home , they will look to you to judge whether the environment is safe. If you sit there with a calm attitude - it will calm them. If they sense that you are upset, they will think there is something to fear so just be as nonchalant as possible and CALM.
I would not confine any cat because they will want to get past any closed doors - cats are like that and they will be overwhelmed with curiosity as to what lurks on the other side.Plus they would be stressed by confinement (which cats HATE). . However, they must always have a safe spot to retreat to if he scares the other .If he prefers one room - that's fine - he may have made it his territory.
If he starts at he girl - just say SHUSH and tap him very lightly on the head. That lets him know (in a cat way) that the behavior is unacceptable and he will understand that feline method of correction and not be hurt, upset or insulted. A cat will only learn from someone stronger that him who is calm, assertive and puts him in his place. As soon as he shows any sign of going after anyone , grab him by his scruff and say NO! in a hissing voice , then gently but firmly tap his head . You must do this every single time you see it.
Do not yell, spray water or act upset, Just immediately calmly and firmly correct him. This will teach him that his aggression will not be tolerated the way another cat would treat him - except the NO! would be a hiss and the tap would be with a paw.. That will learn his respect and he will leave sis alone. Consistancy is the key here.
Just imagine how stressed you become when he goes after her - well he can feel your anxiety and takes advange. A strong cat would never put up with this from him - but your girl is showing him she is afraid and He is trying to rule her
Get a feliway plug in and rescue remedy at a pet supply to safely keep them calm.Use rescue remedy also at a pet supply.
Please let me know how things are going. If you have questions along the way I will always be here for you.
I will help you resolve this so they can get back to being great buds.
Warmest best wishes,