He's not undercover as a servant, he IS a servant. This series is set when he and Arthur are young, Arthur has only just become king and Merlin is just a servant who happens to have special powers which he has to keep secret because he would be executed if anyone found out. Gaius knows about his powers but no one else in Camelot does, including Morgana..although she suspects he does.
Apparently :)
I don't watch it but someone sitting next to me does.