It sounds to me like something inside was leaking and it should not have been!! He was losing blood and this is what caused his BP, heart rate, and pulse to drop. He could have died from this! I think damage was done to the sternum during his surgery, but they will never own up to this. I lost a son due to negligence so things like this infuriate me. I would go to the hospital administrator, don't call, just go and demand answers. The doctors won't tell you everything, they cover for each other, and the nurses the same. I agree with ed, the infection entered during the surgery, or shortly after. Was your dad on any antibiotics immediately after surgery? He should have been, because their is always a risk of infection.
If he is in his own room, I am sure this is due to the infection. They must feel that either he is at risk of infection from others, or that he is risky to other patients. Have they mentioned what kind of infection it is? in particular is it MRSA?
Lifting something heavy would not have caused the infection to occur. This has entered the wound after surgery or during.