Thanks so much for the additional information.
Let me address your concerns separately.
When it comes to burns on a cat's skin, they always look worse before they look better. They can also take several days to develop. It doesn't sound infected from your description and should heal just fine.
I'd gently clean the area with very dilute over the counter betadine or iodine (to the color of weak tea) twice a day. he he can't reach the area to lick off any cream, you can apply over the counter antibacterial ointment such as you would use on a wound for yourself.
As far as the lilies are concerned, however, there are ones which could be quite dangerous although there are others which are quite benign. Obviously it will be important to know which kind you have.
Benign lilies include the Peace, Peruvian, and Calla lilies; these contain oxalate crystals that can cause minor signs such as mouth irritation involving the tongue, pharynx, and esophagus. Signs which might be seen would be drooling, pawing at the mouth, foaming, and vomiting but the signs shouldn't last long.
The more dangerous, potentially fatal lilies are true lilies of the Lilium or Hemerocallis species. Examples include the tiger, day, Asiatic hybrid, Easter, Japanese Show, rubrum, stargazer, red, Western, and wood lilies. Even ingestion of a small amount of these lillies whether the leaves, petals or pollen can cause severe, fatal kidney disease.
If he was exposed to one of these lilies, then I'd advise you to have him seen as soon as possible. Treatment needs to be started immediately!
I hope this helps and that his lily exposure was to one of the benign varieties. Deb