A couple of questions. Is the basking light over the sand? Or is it over one of the Rocks?
150 watts is way too much for a 55 gallon aquarium.
He may be burying in the sand to get away from the heat. Check the basking spot temp with a heat gun if you can.
It should be mid 80's to low 90's. I've never heated my water as long as my room temps stayed in the 70's and they had access to
adequate basking sites. They are fairly cold hardy turtles.
Is the bulb for the plants or the basking area specifically designed for reptiles?
If not it would be a good idea to pick one up. If he doesn't like prepared food, try cutting up a small earthworm for him.
Turtles can't seem to resist the movement a worm makes. And keep trying different prepared foods.
He may actually like a different one.

Depending on how small your turtle is right now, 55 gallons may be overkill. I raise mine in 5 or 10 gallon aquariums
until they are ready to move up to bigger homes. It's easier to monitor what they are eating and easier to clean. Every morning,
I just empty them out and refill them. No filter needed. I know it's tempting to want to go naturalistic, but until you establish your turtle
it may be a good idea to have him in a bare bones tank with a bare bottom and a simple basking spot.

Lastly, a trip to the vet may not be a bad idea. Get a fecal done and have the vet examine the turtle to see what he thinks.
Best of luck.

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