He may be incompetent. Find a new shop to build the wheel. I've rebuilt many wheels with no issue, from replacing spokes to hubs to rims, re-using parts that were 20 years old. If he's afraid of re-using a new rim, he might be scared stiff of liability because he's not sure of his wheelbuilding ability.
On the other hand, are you absolutely *sure* you want to use an Alex rim on your new wheel? Maybe that's the hesitancy the mechanic has...