Why won't my turtle eat?

[Jazzy Fizzle]

I found a turtle in the parking lot maybe about 4-5 inches. I have had it maybe 2 weeks and it won't eat anything. I put turtle food pellets in and dried krill. Also, lettuce, grass, bread, etc. He won't eat just runs away to the corner of the tank and swims nowhere. I think he is losing weight b/c he is so skinny now. Can someone please tell me how to force feed my turtle.
I'm not letting it go. Tell me how to force feed him or he will die.