aggressive cat, need help!

so a year ago i found this little orange tabby cat in my yard no older then 6 weeks. he is a year old now and is neutered. he is always hiding and very scared. i have no idea why because i never did anything to hurt him and no one in my family has harmed him in any way. he has just always been that way. well today for some reason he  went after my German Shepherd. My German Shepherd didn't do anything and tried to walk away but he kept going after him. then my tabby cat turned and attacked my chihuahua Mix. when that Happened my German shepherd went after the cat. (The cat is fine)  i tried to break the fight up and my cat went after me. i don't know what is wrong with him but it scared me. he is a strictly indoors cat. i am thinking of finding him a better home where they don't have any other animals and somebody that will keep him inside. the only thing i am scared of is if i RE-home him that he does the same thing to them and they have him put to sleep. if i do find him a new home how do i make sure they do not do that and how do i know it is the right home? or is there a way i can break him of this? he also attacks my older cat who is also neutered. PLEASE HELP!

This all just happened today?  Then something has happened.  He may be ill and defensive, or it may be redirected aggression. So many potential reasons.

The very first thing I would do is take him for a check at the vets.  Then there are all sorts of things that may help him to settle.. Does he have lots of vertical space? Playtime?  There are Feliway diffusers and other calming aids that may help.

He is probably a shy boy as he left his mum too early.  It's wonderful that you took him in and I'm sure you can get to the bottom of this. Keep us informed of how he is and we'll do our best to help. 

okay ill try my best because it will break my heart if i have to re-home him.