My shower faucet is the type that you adjust the water temp by moving it from side to side. My problem is that after a few minutes the water is cold. I can adjust it for hotter water but after a few minutes it becomes cold again. If I turn the water pressure down the water heats up.
The water in the sinks do not have this problem.. Any help would be appreciated.

Is it a Delta Shower Valve? Can you post a photo?

Yes a photo would help, but it does sound like a problem with the faucets cartridge, it likely need to be disassembled and cleaned.

What type of water heater do you have. It sounds like you have a tankless heater and are running water through it faster than it can heat the water.

That was my first thought as well, but then he said the sinks were fine. Maybe I assumed too much and he hasn't tried the sinks when the shower has turned cold.

Some of the tankless come with a low-flow aerator that they say to install on the sink faucet. So his sink could be ok but shower still has problem with tankless setup, especially if he has an older shower head or has removed the restrictor in it.