While you can go ashore at a tender port IF it is not rough (the cruise line has the right to deny this if weather is rough) they will NOT lift a power wheelchair. He needs to plan to take both a manual folding chair and his power chair. Use the power chair on the ship, and the manual chair for all shore trips. Even those with a dock often are too steep to be safe with a power chair, and you will not find curb cuts or ramps in these ports.
He MUST have an accessible cabin. There is no way that someone who cannot walk can get in/out of a standard room. He should have checked on this prior to booking. Accessible cabins often go first, and may already all be booked. They are available only in limited numbers and not in all categories.
If he is going to Puerto Vallarta, there is an accessible tour company there. We used them in November and were very happy with our tour:
There are none in Mazatlan or Cabo, so he will need to plan on either transferring to cabs (another reason to take only a manual chair ashore) or just see sights around the area of the ship. In Mazatlan the ship docks in an industrial area with little to see. In Cabo it is about 1/2 mile to the main part of town from the dock area.