If he's hung the boiler and "done all the other pipework", who disconnected the gas?
Sounds to me as he has started off on the wrong foot.
I plumbed my boiler in (some years ago I would add) and a GSR did the gas side, but he disconnected the gas to the old boiler in the morning, and came back later in the day to reconnect the gas and run through commissioning. The system was power flushed by me beforehand, and I kept the empty containers of X800 and X100 for him to see.
If anyone had asked questions over it, I would have said I was working under the GSR's supervision - he checked everything (at my insistence) and he was happy to put his name to the commissioning documents.