He is not a bike mechanic nor can he build wheels, I am guessing he cleans the workshop and pumps up tyres, have you asked the shop manager if you can speak to the actual mechanic?
On the other hand, if he is the mechanic you should name and shame, there are too many cowboys geting paid by people that deserve better while other people interested in doing that job are working in tedious mundane locations.
He should have been able to calculate spoke length by measuring the components, and three weeks is plenty of time to obtain any size not in stock!
Three weeks is plenty of time to train a few mechanics in wheel building and with time to get some hands on!