The cannister he's alluding to is the charcoal canister. The valve he's alluding to is the purge valve.
As the gas tank breaths out, when filling the tank or for any other reason the volume expends (like when agitated), the gas fumes are collected in the charcoal canister. When you start the engine, the "purge valve solenoid" opens the "purge valve" and lets the engine's vacuum draw in the fumes collected in the charcoal bed. Either one of these components has failed or the orafice through which the fumes are drawn is clogged or gummed up.
It sound like your personal mechanic knows of what he speaks and can troubleshoot and repair the system. A dealership would have no advantage in this scenerio. But they'd probably charge more.