Boy, you are not kidding about the weather report for St. Louis!
My nephew just went there last fall for college and he has really had some weather since then! Hope you're warm and cozy!
Aw, lucky little guy to be rescued! I think if you feed a good food (like Nutro), you probably don't have to supplement--esp. since he's a year old now. That would be my first thought. Vitamins certainly can't hurt, but since their food (the good stuff) is formulated to be almost perfect (unlike what we humans eat), I think he'll be fine.
What's his name?