Yoga, even at it's most powerful does not raise the heart rate and sustain that raise for the training of the heart and lungs. Yoga tells us to be responsible and active in our own lives and in the past that meant walking around and actually doing things. Heart disease kills more people every year than all cancers put together so this is not a subject to be taken lightly. Flexibilty training loosens tight muscles because tight muscles pinch off circulation and nerve messaging and limits the ranges of motion at the joints. It is very important to see a health care professional to rule out any other sort of problem.
Some people relax in the evening together.
Also be sure he is getting enough water. Not enough water makes the blood thick. Thick blood doesn't move through the brain well. This sounds like nonsense. It's a previously proven phenomenon.