Your father is a Diabetic and he got his left leg amputated.
He is also suffering from bacterial infection for which he is taking antibiotics.
Now he has developed a small blister
in the stump.
This is significant and should not be left untreated.
The overall mortality of a diabetic amputee is higher due to various reasons.
Check for any pressure sores
Check for any underlying ulcer.
Provide proper wound care.
Revisit the doctor.