Not appropriate as a greeting. He has no sympathy. He says "I sympathize with you during the heat♪", but the exclamation note at the end of the sentence is a musical note(♪) which represent playfulness, so it means that he is glad that the recipient is in the bitter heat of summer. Besides it, the picture is a scene where the characters are having fun playing.
But in the first place, it is a mistake for erotic illustrators who have not studied at all in school to greet someone, huh? They only need to paint, but lately that hasn't been enough. Any Illustration styles has modernly and pathetically become something to please otaku who are different from the general public. Do you want to hear what they are? They are usually given a high rating average here.