Mike, how long ago were you a certified A/C technician? We non-certified A/C technicians can't buy R12 anymore.
I'm sure you're right that he's low, but he will have to take it to a station to get it charged. Another option would be to get a kit to convert it R134a. I did that and mine blows nice and cold. The cost of the refrigerant is also much, much lower.
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351W 89 Mustang GT Convertible