Bit of an urban myth that one, I'm afraid. If he had a heart attack with an erection, the blood would simply have been reabsorbed back into the systemic vessels within a few minutes. Blood flows into the penis during arousal due to the inflow of electrolytes from the arousal centre in the brain that widen the vessels and allow blood to flow in. The penis becomes engorged with blood and, hence, la bon stiffy. Upon death, the electrolyte supply would have stopped, the vessels would have contracted and the blood squeezed back out.
No doubt too much information...!