Low Ejection Fraction

My hubby a 43+ yrs old had had a massive MI in May and was found to be having an EF of 40% (MUGA Scan) after the stenting. His LAD was 100% occluded prior to stenting. He had been following a cardiac rehab program and had been very good to him. However, the EF last checked last week showed an EF of ~38% (Echo). We both are worried. Does this mean that his EF is not going to be improved? Is he over exercising? He does not feel fatigued or having shortness of breath or anything like that. He has been eating only Healthy.

Can we please know what is his optimal heart rate for exercising? Will he be able to get his EF improved? He is on plavix, Ace inhibitor, beta blocker and other meds.