that's an not acceptable form of UVB, the UV light from the sun that actually GETs to the turtle is minimal if any as it is filtered by the screen, glass, tank, etc. ALL turtles should have a 10.0 or 12.0 UVB simply because the 5.0 UVB barely penetrates the water surface. I bet $$$ his medical issue is because of a lack of UVB, which prevents them from producing Vitamin D3 which THEN prevents them from absorbing Calcum regardless of how much you give them, so this would be the perfect explanation for why his is having shell issue. Calcium is essential for turtles shells and strong bones, and I'm afraid if you don't get him under a UVB, it will lead to a severe calcium deficiency and eventually Metabolic Bone Disease - a horrible painful way for an animals to die, which is incurable and disfiguring.
Whoever told you putting them "next to a window" was good enough UVB should be beaten senseless with a wet noodle