Tiny boy underwear??

So this is super random but I need help finding really small underwear for boys. My son is 2.5 and he is tiny and all of the underwear I have seen look as though they will fall right off him. And when I say he's tiny I mean really small. He's 22lbs, 8% percentile for height, not even in the growth chart for weight and wears 3-6 month shorts. We'd like to start potty training soon so underwear is a must. There has got to be some company that makes really small toddler underwear, right?