I am SO sorry! I HATE that phase. I've had 3 screechers so far. My first started it about a year old seemingly just for fun. She would do it just to see what reaction she could get. It was excruciatingly painful for us, but we learned to just ignore her when she would do it, and overly praise her or act happy and proud when she wouldn't do it. She grew out of it rather quickly. It only lasted a couple of months.
My twins both started the screeching at about 7-8 months old. They would do it for every reason. Just for fun, when they were upset, tired, or just for no reason at all. They did it every day multiple times a day. Unfortunately, the ignoring didn't work with them because it wasn't always for no reason. We ended up just softly saying "No." when they were done screeching for the first screech or two, then just ignoring them when they would do it for the reaction. They still haven't completely grown out of the phase yet (they're 1 1/2 now) but they don't do it nearly as often. Usually only when one of them does something to really bother the other one.
My 6 month old might be starting that phase now too. But she only does it when she gets upset. It's no fun at all, but I haven't figured out a cure for it yet. Sometimes, I just put them in another room (as long as the screeching isn't a result of some need they truly have) to give them some time alone with their own voice. Good luck with yours. Find what he responds best to and stick to it!