I fully understand your concern.
The vast majority of PFO's need no treatment whatsoever and usuall close spontaneously. I would have to say that his blue lips may be from the PFO but really the only way to be sure would be to have an oximeter on his finger at the time to see if his arterial oxygen leve were low, and since he otherwise seems fine then really I don't think oxygenation is a problem for him.
I agree with another scan but PFO's don't enlarge and don't cause other heart problems so he doesn't need surgery
Let me know if you need any more information
Yes, the previous scan would have show any other heart defect or problem and most often a PFO is the only defect ever seen.
And yes, since he's otherwise fine with no shorness of breath, dizziness, balance problems, etc there really isn't anything else to worry about here.
And please don't think that the other case has anything to do with your grandson's PFO.