Hello and thanks for the query.
I will advise you to consult his doctor to see whether he has any diabetic Nephropathy
function tests and total protein in urine
must be checked up since in long standing diabetics,chances of diabetic Nephropathy is high and the dose of anti diabetic drugs has to be brought down.
If he is found to have Nephropathy then you should consult your doctor to switch to injectable insulin
instead of oral anti diabetic drugs.
Thanks and best of luck.