I would have him pull the tank covers if possible and inspect the tank straps and also that gives you access to see other problems. Wayne W. lost a plane due to corrosion when he had his removed, i.e. they found corrosion there.
But, I would have him look at the service manual for the 150 and do the list of things in it. It's basically an annual and if he'll sign off on the planes annual using that checklist and you know there's no corrosion on the spar then you're good to go.
Like my A&P says, the 150 is a simple plane and easy to see what's broken most of the time. No strange contortions are necessary to get access to stuff, i.e. looking at the nut plates and bulkheads for cracks in the rear.
