Truthfully, I think that it is time to stop trusting your mechanic.
His alignment equipment may be in need of calbrating, or alignment might not be his speciality, or–even more likely–you have wear issues with front-end components that he is not noticing.
I suggest that you seek the best alignment shop in your area. Take the vehicle to them, describe the problem, ask them to examine the front-end carefully, and allow them to give you their diagnosis.