Hi OP,
Previous community nurse here. Please see if you can talk to his district nurses on his behalf.
Unfortunately ulcers at this stage, on a smoker with poor diet and other serious health conditions are very unlikely to ever heal. The cause is related to his circulation, the wound is a result of that.
I would really advise against buying dressings online.
Manuka honey in particular can cause an increase in wetness and macerate the edges of wounds causing further breakdown.
Silver impregnated dressings tend to be ineffective in leg ulcers.
I wouldn't necessarily assume it is infected. Surrounding skin of venous ulcers is often very red, inflamed looking due to poor circulation. Have his nurses/GP seen the pictures?
The only real treatment at this point would be surgical, which they absolutely wouldn't do with him having stage 4 cancer.
What he really needs is a Doppler and proper compression bandaging, however if he won't accept this:-
-Keep the elevation as much as possible - so him lying down or fully reclined in a recliner. Just having your foot on a stool is not enough, foot needs to be above heart level.
-Keep it as dry as possible. His nurses should advise on best dressings to do this and frequency of changing. If bandages have any strike through(fluid that has come through) change them straight away.
-Improve diet at much as possible, he will be losing a lot of protein. Ask GP about fortified drinks.
-If your dad would be willing to re-engage with the hospice team (over the phone) they would be the best people to advise about pain relief. Venous ulcers are often not as painful as they look, thankfully, but there are lots of options to help discomfort.
Horrible situation, I really feel for him.