Burning is highly effective, although it does leave scarring and is typically used as a last resort. Cryotherapy (freezing) typically does not leave
scars.
Aldara is effective and doesn't typically cause scarring, but seems to be moreso effective in females than males I've read, and TCA acid also doesn't cause scarring if done correctly. There are an array of treatments that do not cause scarring, and if your son hasn't begun treatment yet, I would urge him to do so.