J.S.
I really like the suggestion to check out the colic clinic. Definitely rule out any medical problems....any chance he's starting to cut teeth?
If he's truly quite well, and seems to be shrieking for fun, he may just be enjoying the sound of his own voice! LOL. You can play with him. When he screams, make the same noise back at him but MUCH quieter...like a whispery scream. This will likely completely delight him and with any luck he'll start lowering his own screeches to a less unpleasant decibel.
Both my kids went through a screechy phase. They were a bit older, but it was clear they were just enjoying figuring out what their voices were capable of. Good luck!