The soul of any portrait from a pastel drawing to acrylic on canvas I always start with the eyes. They are what makes the portrait feel alive and its the main part that I feel makes that pet look like ‘Henry’ rather than ‘a Cavalier King Charles’.
Even darker eyes like Henry here there is still so much depth to capture. The subtle colour with the highlight at the very bottom and the bright highlight is what will make the eyes look alive. Getting all the areas where the light is captured and then the pinker area in the corner are all vital.
There are times where the reference photograph is perfect in every way but the eyes may be a little dark and in shadow. In this situation I like to have a few photos that may show the eyes clearer. I then tend to use details from these photos and incorporate that into the portrait enhancing from the current reference you have.