Rescued kenyan sand boa, what might be the issue?

[legion418]

I recently took in a kenyan sand boa from a friend who did not really take care of him. I'm told he is about 3-4 years old. The problem is is he's probably only 6 inches long to me, and extremely skinny and small head. They fed him very small amounts once in a blue moon..and his uth that they had used was a heating pad, never water or definitely not clean water, and they had used sand. I switched him to a tank with a correct reptile UTH and a low wattage heat bulb creating about 89 degrees, with eco coconut fiber. They used to feed him live pinkies...I switched him to f/t, and he ate 3 pinkies first night I had him, and he definitely looks better.. but any tips? i'm normally a ball python guy. I know sand boa's normally stay smaller.. but for him being supposedly 3+ years old, I don't believe he should be this small still. maybe impacted from the sand.. and they're random small erratic feeding?