It looks like it is because of phrenic nerve injury after coronary artery bypass grafting ( CABG)
Normally Phrenic nerve injury after coronary artery bypass grafting resolves in most cases. But it has been seen that
a significant number of patients who incur phrenic nerve injury after coronary artery bypass grafting have persistent phrenic nerve injury.
Hiccups are involuntary contractions of the diaphragm and as phrenic nerve is the one which controls the diaphragm so damage to the nerve leads to this problem.
Some medicines can help but it can't be prescribed on tele consultation but i think you must have already tried it as it has been 2 years.
If medicines not working then there are some non pharmacological treatment , nerve block , radio frequency ablation etc
Next Steps
consult a neurosurgeon if possible for you.
Health Tips
Try to notice the trigger factor like some food or stress or anything.
avoid alcohol
avoid carbonated drinks.
Avoid spicy food.
Avoid over eating.
keep him hydrated.
hope it helps.