When faced with such a situation I would personally go with a custom 1-up sail that is cut a tad fuller (say for a heavy singlehanding person). I figure your 11year old doesn't weight very much so you won't be anywhere near 150 kg (330 lbs) and otherwise be depowering (the 2-up mainsail) nearly all the time.

After several years of your son gaining weight, you might just be in the market for a replacement sail anyway if you sail often during that time.

Wouter