I agree, you have to just ignore it when you can. He realizes that it will get him some sort of attention, and a lot of times at that age they are learning social skills - like when I do this, she will do that - will she do it every time? Is it good or bad? That kind of thing. If you can ignore it, do, and if it's happening at an inappropriate place or time, just say ssssh, and use your inside voice - that kind of thing. Since you are only watching him, you may want to come up with a plan with the mom, so the word you are using is consistent - sssh, quiet, use your inside voice - that way he will learn that some place he can scream, and others he can't. Also, most of the kids I know, including my oldest two, went through this phase. They grew out of it as they learned more words. My 8 month old is gearing up to be the loudest of all three - he loves to hear his voice, lol.
Good luck!