the answer is is 50% because if the disease is a recessive trait, his grandparents could've gotten the gene of the disease from their parents, so its 50-50 chance that his grandfathers are carriers, but it mostly depends on what genes their mom and dad gave the grandfathers, and then they probably passed the gene on their kids, and then their kids passed the gene onto fred, and he ened up with the disease.
hopefully this helps you