Why does my bf have pain in lower lower back, watery stool, and nausea?

he woke up and puked 3 times.. i thought maybe it could have been kidney stone but wear it hurts on his back isn't near wear the pain should be... and it doesn't hurt when he urinates (or bleeds) but he said he has very watery stool (poo) it happened this morining so it wasn't constant pain. (the last thing he ate and drink, last night, was gummy bears (ugh) and a milk shake thing... i'm worried...