My pit Bull mastiff has always been gentle. He is 5 now and becoming very aggressive towards any new person?

He is large male. Have had him since 2 weeks old. He was attacked by his father at 6 months, over female pup his sister. Father had to be put down. Never a problem kids coming and going from home all the time. Two years ago he had to spend 7 days at the dog pound as he scared neighbor, then released, but just wasn't the same. Now he wants to attack anyone new or who rarely comes by. Is there any thoughts, It breaks our hearts to have to put him down. I fed him from a bottle when he was a baby, When he was attacked by his 145 lbs father, I had to cover his body with mine, any slide his bleeding body to safety. The father would attack him if any part of his body that I wasn't covering, but never tried to touch me. HELP ???