It's really sad that his diabetes
got unnoticed for such a long period.
Now, for the best outcomes he needs to control his blood sugar as normal as possible and if with current medications it is not coming under control then please do not hesitate/ refuse for insulin therapy
. Insulin helps in healing process, bringing down inflammation and lowering risk of further DVT.
You did not mention his age, height and weight. You will need to see a good Diabetologist
for appropriate prescription of medications to control the sugars further.
I hope his blood pressure is also under control and he is taking all anti-coagulation therapy as advised by his doctors.
For any further query please get back. Take care.