There are a number of causes that can cause weakness/fatigue following a major cardiac surgery which can include anemia, infection, medication side effects (most often beta blockers), and general post-operative fatigue from deconditioning in the hospital. If he previously felt well after his surgery and now is experiencing new symptoms, you should make sure he is followed up with his cardiologist to ensure his labs and medications are in order appropriately.
I am not a doctor, but I do know these surgeries can take a toll and recovery can take quite a bit of time. With type 2 diabetes you can also have healing issues. I have 25 stents at the age of 44 (4 heart attacks from 38-42, no more stents since 42). I have had my adrenals checked using a 4 times per day saliva collection that insurance is now paying for (at least mine is), and also had my thyroid checked as I found I was very low on testosterone and if you have thyroid or adrenal issue, testosterone replacement therapy may not work you. Adrenal fatigue - caused by stress both physical and mental - can cause general and sometimes profound fatigue. The same with hypothyroidism and low androgen levels. I would look for a good doctor of osteopath who will run these tests and see if there are any issues with either of these glands. Also, many open heart patients suffer down the road from a delayed depression, which can also lead to fatigue and apathy. While I am not a fan of SSRIs, there may sometimes be a need for this. My mother-in-law had depression issues that started about 6 months post-surgery. The operation itself is amazing to me - that they can even do something like this is a testament to modern medicine. But the hit the body takes is heavy and some people have a tough time recovering from that. Finding the underlying system that is being affected can go a long way towards treating the issue.