How do I put my mentally challenged son in foster care?

My son is 19 and has the brain of a 3 year old. Taking care of him has been very hard. He is non-verbal, not potty trained, wipes his wastes all over himself and can't be left alone. He is learning to read. He can read simple words such as cat, dog, mom, dad, etc but it's hard to understand what he's saying. He can read simple sentences such as "the dog chased the cat" but when he communicates, his sentences don't make sense. He can count to 60. Physically, he is stronger than me. I have scars all over my and body mostly my face and my arms because he scratches me. He will fight me and I don't have the energy to fight back. I also have a 17 year old daughter, a 16 year old son a 15 year old daughter and a 3 year old niece whose parents are overseas. I would love to spend time with them but I can't because I have to take care of my son. I want to put my son in an institution/adult care but I panic because I'm afraid that something is going to happen to him. I lost my father to medical malpractice and I heard stories such as a 10 year old girl killed in the hands of 2 therapists and a 2 year old killed in foster care. My house is not suited for him and I can't take care of him. How do I put my son in a foster care/institution I can trust?