"Scar Tissue" vs. "Scar Tissues"
When referring specifically to "tissue" in the biological sense, not "tissue" as in the paper sense, the rules for usage are as follows:
Use "tissue" when referring to a specific type of tissue. For example, when referring to "adipose tissue", it's "adipose tissue", not "adipose tissues". Example sentence: "Adipose tissue can be found in various parts of the body, particularly around the waist."
Use "tissues" when referring to several types of tissues. For example, if referring to an area of the body where they would be several different types of tissues, the plural is used. Example sentence "The fire damaged the tissues in the left leg of the animal."
Thus, in your sentence, I would say :
He applied ointment to the scar tissue.
Because he's only referring to one type of tissue.