If a piano accordeon tech can't diagnose, identify and correct a diatonic reed problem, I wouldn't let him touch my piano accordeon. You've go the wrong person.

What city ? Perhaps someone can give you a referral to a competent tech.

Have you opened the box and looked at the reed ?
This is easy and need not be feared. I am guessing you can fix it yourself.

Have you depressed the button and vigorously pumped the bellows to see if this condition clears itself?

Brian and others here can coach you to open the box and isolate the reed. What you describe is not necessarily a busted reed... unlikely.

Re-reading your post... your tech doesn't know where to get a reed or how to install it? You must be joking. Elemental my dear Watson.