Welcome to HCM,
Your father is having numbness in feet, which is symptoms of Neuropathy,pain was due to sciatica, he has be operated for coronary artery disease, I presume he must be taking all treatment.
His MRI is not showing any compression, then neuropathy may be caused by nutritional deficiency
like B12,other causes includes alcohol induced,any drug induced if he is taking,inflammatory pathology.
He should visit a Neurologist
and get Nerve conduction study
You have mentioned he is not a diabetic, once cause of neuropathy is detected it can be treated.
In the mean time take good care of feet and treatment like neurotropic vitamins, Pregabalin
may be given to see if gives symptomatic relief.
If his pain is severe he may get good relief with Epidural injections.
Please take care.