you can't "extend the fork" on a BMX bike like
alo suggested. not sure what that means. also, are you suggesting he put a wheelie bar on it? that's a bizarre suggestion.
check where the axle is in the dropouts. if you can shove it further forward by shortening the chain with a single half-link, or slightly changing the gear ratio, that will make the back end shorter and therefore easier to get your weight over the rear. that may not help you maintain that position, but it will be easier to get up there.
handlebar position is a big deal. start with the upright part of the handlebar parallel to your head tube. adjust for and aft to your liking, but it should be pretty close to parallel with the fork.
learn how to manual. wheelies are a half-assed substitute for a proper manual.
are you sitting down when you do this? I never learned how to sit and wheelie, probably because I always rode BMX with a very low saddle.
high pressure in your tires might help. if they are squishy, the handling will feel vague.
watch some "ride out" videos. those folks can wheelie.