The blocks in the vessels which occur in diabetics are usually widespread and cause problems in more than one place, so surgery is often not sufficient in restoring blood flow.
i) I presume he is on low dose aspirin, and perhaps even clopigrel, to thin his blood.
ii) Exercise, to the maximum extent possible, is very helpful in opening up other blood channels, and so improving blood flow.
iii) Good control of diabetes is very essential: an HbA1c just above the normal level, frequent blood testing at home at different times to make sure the doses of medicines are adequate.
iv) Tight control of blood pressure is very essential: make sure his pressure is as low as he can comfortably manage without symptoms. Ideally, BP should be 110-120/75-80 mm Hg, but somewhat higher levels may need to be accepted if he gets symptoms when he goes to this level.
v) I hope his lipid levels are controlled.