The C&G Level 2 diploma that he will be doing in college is what's classed as a technical certificate, that gives the grounding in all the essential plumbing technical knowledge with working practices, site skills, water standards, Hot and cold water design, pipe skills, etc. That then sets him well to start an apprenticeship and work towards his NVQ. This is done in the field with an experienced plumber and he builds a portfolio, reporting, gaining evidence up to a prescribed level as required by the qualification and then receives the Level 2 (S)NVQ , that then allows him to call himself a qualified plumber.
If preferred then once he get his LeveL 2 tech he can then continue and do his level 3 Tech Diploma - Advanced water/Gas/etc and then straight to the level 3(S)NVQ through the apprenticeship but as suggested it does require a further course at college or training centre.