This is one of several glitches in Ryanair's software. In effect you cannot check your son in online and must do so at the airport (but check everyone else in online). When you get there explain the situation to the Ryanair sales/info desk. They will direct you to the check-in desk who will check your son in. Ryanair's staff are almost certainly familiar with this position but if not they may indicate you must pay the manual check-in fee. Should this happen you should stick to your guns and ask them to demonstrate to you how you can check in online. My guess is they will check your son in without a murmur (I had a different case but the result was the same and it all went fine). This is one reason why Ryanair's communicationless setup is irritating - and these drip-drip fees, traps, etc. are too.