Mallary by Matthew boys Cotton Briefs Underwear Classic

My son is growing out of the standard size 6. not sure what size to buy the M or L? 10 seems way too big but in standard sizing 8 is the next size up.

There's no box for size selection on this page; how do I order size?

Son is growing out of size 6 underwr, so i was going to choose the medium 6-8, but then saw their chart & per his 95 lb weight i should choose xl??

Should I base my sizing on waist size or weight if they don't align with the same size?