What are the texts/messages like? Does he ask you to hang out? Does he ask you to hook up? Is there any inappropriate language going on? What exactly do you get out of receiving messages from him? Do you like the attention? Do you enjoy your friendship? Or are you hoping he changes his mind about his relationship with his girlfriend so that you two can pick things up again? If you feel uneasy about the messages, then it's my best guess that his intentions are not to be "just friends", and you are picking up on that energy, but you don't want to give up the skewed relationship you have with him.
Do you truly believe that you and this guy can be just friends, while he has a girlfriend? If you do, then do that. Be aware of your feelings, and the situations you allow yourself to be in. If you feel that he is playing you and his girlfriend, then I suggest you cut contact with him until he can learn to balance a friendship with a woman, while being in a relationship with a different woman.
"Caring is not an advantage."