The magic of his blood aids in healing, so I have some leeway in terms of how often they can take blood and how much it affects him. Needles have been developed to improve blood extraction, though they are larger than normal hypodermic needles, more similar to the needles used to drain fluids from joints.
I've done searches on the affects of giving blood, the longer term effects of repeated blood loss, the effects of anemia due to iron deficiency, and iron-rich foods.
What I'm really trying to nail down is:
1) What sort of state would this person be in? Would he be constantly fatigued? Would his mind be gone? Would he be in any sort of pain?
2) I need for my MC to try to help him in some way. I had originally thought he might give him something to ease his pain, but if he's not in pain, I'm not sure what to use.