Hello and welcome to HealingWell. Your Dad has had a double whammy - a heart attack and the triple bypass surgery. He is also 76 years old. To make a full recovery from his ordeal it may 6 to 12 months before he feels 100 percent again.
If you are not satisfied with the Drs. assessment of your Father's weakness, please do seek out a second opinion as it may not be related to the heart. Also sometimes medications can cause spells of weakness.
I admire you for caring for your Father and seeking out help.