When you say 'heaviness' can you be a bit more descriptive? Do you mean it still aches? When I had a triple bypass is took a year before the aches stopped, some people I met at rehab that I keep in contact with had aches for a while longer, so everyone is different. I assume you have been having follow up appointments with the cardiologist?
With regards to the grafted vessels, they can block, just like any other vessel, they are not immune to artery disease. However, there are lifestyle changes which should have been made to help lower the risk. Measures such as cholesterol control, healthy diet, regular exercise, no smoking, blood pressure checks and control.
If he is getting chest discomfort upon exertion or shortness of breath, I would see a cardiologist sooner rather than later.