I just had this exact issue as the OP with someone who has never had an iPhone or any other Apple product. His phone number is not tied to any AppleID. So the excuse that it says it's "delivered" because some other Apple device is picking it up is not true. He doesn't have an Apple device. The problem lies with my iPhone. In case anyone else comes here looking, what eventually solved it for me (which was super annoying) was to delete our entire conversation history and have him send the first text to start us off again. Then, when I replied, it finally sent as a text.
Update: this occurred only after updating my iOS to 13.5 on an iPhone 7.
Update: this occurred only after updating my iOS to 13.5 on an iPhone 7.