What's the arrangement on these jobs Matchless?? Are you just giving him the work and the customer pays him directly or are you subcontracting him in for the gas work (nothing illegal about that). If it's the latter, in law you are responsible as well if he leaves any defects so be very careful.
I went to another gas fitter about half way through my portfolio after it became clear the one training me was, shall we say, not very knowledgeable.
I haven't bothered to do the pipe sizing calculations but what I will say is the tables are quite conservative already. As long as the figures come within the allowable values in the table it should be fine. Clearly there are other issues with this guy too, I'd be tightness testing pipework on completion of any boiler installation.