Snappers will bask more when ill in order to raise their body temp. This helps fight the illness. Maybe he has an infection or something.

Can you hear him breathing? Are there bubbles coming from his nose? Is he eating? Are there any new marks on his shell? (cuts, scrapes, red lines, white dots, etc.)

You can try raising the water temp a bit, but you might want to take him to the vet.
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0.1.0 ornate box turtle
2.0.0 eastern box turtles
1.0.0 Yellow belly slider
0.1.0 Red belly cooter
0.1.0 Australian shepard
1.8.0 chickens
1.0.0 Dutch rabbit
3.2.0 adult children
1.0.0 husband
AND
0.1.0 grandbaby!!