I wonder if contracting other diseases along the way had somehow contributed toward his resistance, as long as the immune system has recovered since. The plague stopped rather abruptly in the Middle Ages either due to sudden herd immunity of the survivors' immune systems or other unfavorable circumstances to its propagation.

I imagine there exist a portion of the population with a genetic mutation that is resistant to Ebola, considering it may have existed in the wild for a much longer period than its first description in the mid 70's.