Sweet potatoes are pretty rich. I wouldn't offer it any more than once a week. In between times keep offering him the green leafies - they'll eventually be his staple. Turtles consider anacharis and some other water plants to be quite tasty too. My turtle loves fresh basil - he would eat it before he'd eat anything else green. He absolutely loves sweet potato too, by the way. Isn't it odd - in nature a turtle would never ever get to eat a sweet potato.
I give lightly-steamed sweet potatoes or yams every week to week and a half (it depends on what's on hand). They're good in that they're high in beta carotene (Vit A).
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-