If it is truely occassional, then realistically I see little potential harm. Just because he's 14 doesn't suddenly mean drugs are going to ruin him and i'm rather certain that MDMA is a pretty safe drug.
Look at all the teenagers binge drinking weekly & smoking tobacco. Alcohol is a powerful drug with potential for addiction, adverse health effects and, to a lesser extent, brain damage. Nevertheless, lots of teenagers abuse the drug weekly & still become successful and healthy. Taking substances during maturation increases the susceptibility to damage, but it doesn't mean he will definitely experience anything negative. I suppose the true dangers lie in the fact that they are young, naiive & feel like they're invincible so completely overdo it. Of course, the right decision, and advice, is to wait till you're 18 and feel knowledgable about what you're doing. But, prohibition never did work. So if he's hellbent on doing it, have a look around this forum for advice on how to do it as safely as possible. The 'Six Simple Rules' thread (situated at the top of the page) is a great place to start.