My son fills his mouth up with drink & lets it run out everywhere?

My 23 month old son started doing this about 5 months ago & then stopped. But the last two days he has started up again. He fills his mouth up with drink & he either spits it out or opens his mouth & lets it roll out. I'm sick of giving him 3 baths a day & changing wet clothes. On top of that he did it while in my bed last night with his cup & I had to wash my sheets, ugh. Do I really have to stare at him the whole time he's drinking now?