Welcome to HCM.
I very well understand your state of mind when confronted with such a situation. Of course the best option is to call 911 and if you have neighbours, call them over to help you manage the emergency situation. In such emergencies two or more brains are much better than one and a better decision can be taken based on experience and knowledge.
definitely needs medical attention at the earliest. If circumstances delay the provision of medical assistance, best you could do is rest the knee in a position which is most comfortable and avoid unnecessary movement of the joint.
Wishing your dad all the best.