You didn't answer whether you had cleaned the paint splatter off? I get that it's annoying, but if that had happened to me and I'd cleaned it off, I would probably mention it so they would think to be more careful next time, but I wouldn't deduct money from the bill. If it's really bad and you can't shift it then get him back and show him.
And have you cleaned the paint off? If so then although it's annoying you've suffered no loss and you haven't a leg to stand on to make a deduction. Generally if you have something you want protecting, sort it out yourself before someone starts work or make it clear you want it protected.
I'm considering knocking money off my final payment to him as I have spent an hour on my hands and knees cleaning dried paint from my new tiled hearth and kitchen floor. It appears he covered up the horrible floorboards that we are going to cover anyway but left the hearth unprotected!