Hot water insufficient

My son is having a problem getting enough hot water. He has an older gas water heater that had been working fine until a couple of weeks ago, could not get enough hot water without turning the temperature control all the way up. I'm thinking it may be the thermocouple ? I had a similar problem a few years ago but it turned out to be a leak in the hot water line under the slab. I have also had the thermocouple problem as well.

It's not a thermocouple...

It could be another slab leak is the water heater running almost constantly even when hot water is not being used?

it could be a bad t-stat as well

What is the temperature of the water when first used? Is this electric or gas? If it starts out okay and quickly cools off, it could be cross-over - the dip tube could be broken up and essentially no-existent. How old is the unit?

Could be a bunch of stuff...
But not the thermocouple...

Thanks for all the suggestions. Whatever the problem was, it has stabilized and he does not want to pay someone at this point to inspect it.