Just put some grit into a separate bowl they will eat it as needed. I'm pretty sure duck weed has alot of nutritional value but he also needs a complete feed, try making a warm mash out of a small amounts with a teaspoon of ACV added to the warm water then mix into his feed, maybe making it...
When feeding anything but crumble always offer grit. Okay sounds pretty serious any chance of a vet? You are offering what for support besides the greens and cracked corn. Cracked corn is basically like candy not too much nutritional value. Do you offer a complete feed as well?
Most Walmarts carry vitamins but once you start him on the Brewers yeast it can take 2-5 days before you notice improvement.
What are his symptoms please?