So, you're obviously capable of walking around in fear, if you're running and hiding in another room. What I would like you to consider is that this is a choice on your part, and that you are capable of walking around confidently knowing you can handle him. Because he is a cat, and weighs, what, 12 pounds? If he goes after you again, grab him by the scruff of the neck and disengage yourself from his claws by moving whatever part he's clawing down towards his tail and then away. Talk soothingly and in a high pitched voice. Keep your eyelids droopy - the sleepy bedroom eyes look - this is calming to cats. Now you have two options - the slow release or the toss release. Slow release is put him up on something high - bookshelf, cat tree, bathroom counter - and slowly relax your grip while talking to him. If he struggles or immediately tries to bite you, go for the toss release. The place to do this is onto your bed, sofa, or stuffed chair. I'm not talking about throwing him, I'm talking about a gentle toss with one hand on the scruff of his neck and one supporting his butt, about 2-3 feet from the chair/sofa/bed, just far enough from you that he can't turn around to re-engage.Now every time he growls (which he never used to do, by the way), I run and hide in another room. So frustrating! We used to be like this --->
Keep a little container of catnip or treats on you. Next time he growls, talk in a soothing high-pitched tone, keep your eyes relaxed and partway closed, blink at him (cats do a slow blink at each other to indicate trust), and offer catnip / treats. Try this first just standing still. You may or may not get better results by sitting down and doing it, but don't try sitting down until you've got some kind of consistent response out of him with the standing calmly routine.
Has your boy interacted with visitors before? Have a friend over for awhile and see if there's any change from previous visitor interactions.
I'll re-iterate finding someone to house your kitten for a couple of weeks (at least 3, I would think) and seeing his behavior changes. Try separating the boy and the girl for as long as you can each day, if you can't find someone to house your kitten. This will require locking one of them in a room (with a litter box).
Lock him in your bathroom in order to vacuum your bedroom. Or lock him in a carrier in the living room (may be less stressful, vacuum monster not right outside bathroom door, if your bathroom abuts your bedroom directly)
How many litter boxes do you have? Two is the minimum, three is better.
Do you live in an apartment? Do you trust the apartment complex to notify you when they've been in your apartment? Have any maintenance people been in your apartment recently? Have neighbors recently moved in or changed in any apartment adjoining yours? Are there any noises outside your home that have changed recently (construction, idiots playing their car stereos, idiots revving motorcycles or cars for no reason, even kids playing and screaming)?
Has he ever shown any aggression towards the kitten?
Are you able to trim his claws (do you do so on a regular basis)?
Is he eating and drinking and peeing and pooping normally? Vet may have missed something. As in, you need to actually watch him use the litter box.
Do you have any tall cat trees / places he can climb up high?
What's his name?
