Chances are that your son has the real thing Ben.
1. The overwheming majority (if not all) of fakes have a pretend 18K nib as it increases their chance of making few more bucks.
2. The seller "looks" legit and pen-aware (although this is not 100% guarantee - I know of one e-bay seller with tons of pen transactions that buys and resells Sonnet fakes)
3. The only other possible tell-tell signs are the boxes (check:
http://www.fountainpennetwork.com/forum/in...showtopic=18572 if you have not done it yet) and cosmetic defects at the joints (see
http://www.fountainpennetwork.com/forum/in...?showtopic=6940)
AntoniosZ