It would be a lie, but he could also claim to have donated last month. Blood donors are limited to donating no more frequently than every 8 weeks.
In polite company, "I can't" should be sufficient. If pushed, "I'm not eligible to donate" should be the end of it, and he would be completely within his rights to not provide any further details. In all probability, he won't be the only one at the Planet who can't donate.
It is extremely likely that one of the articles they will be researching is one appealing to the population of Metropolis to donate blood. Such an article would include details on who is and who is not eligible to donate. It would become clear to everyone that not everyone will be able or eligible to donate.
Those who can, should. Those who cannot, should not be made to feel guilty, nor should they be singled out for attention in any way.
