With asthma or any respiratory disorder the lung exchange of carbon dioxide for oxygen is hampered, and the right ventricle is pumping against higher resistance and the heart needs to increase the cardiac output. The overwork causes the heart to increase in size. At first the enlargement in size will increase contractual strength which is compensatory, but if the underlying cause for the enlarged RV is not properly treated The RV will enlarge to a size that detrimental, and this will decrease contractual strength and eventually this will effect the left side of the by not supplying enough blood/oxygen from the lungs.
The volume of blood should be equal on both the left and right side; so the left side and the right side should have equal ejection fractions (EF) to maintain a balance. An EF of 50 to 65% is considered normal. The measurement is not static and varies some to maintain equilibrium as does the heart rate. There is a margin of error of about 10% of the estimated value. So your son's EF is marginally low.
Yes, exercise can help, it will strengthen the heart muscle causing it to pump more effectively. More efficiency of the cardiac system will increase the EF. Bronchodilator will help the heart as well as it will provide less resistance for the heart to pump against.,,, more oxygen with less work for the heart.