My head is telling me to back off but my heart is telling me to stay?

I met a guy who is a real gentleman, one of those rare guys that hardly exist anymore, and well he has a terminal illness and has been told he has only 3 years to live, my head is telling me to back off but my heart is telling me to stay with him, i know that if i do i am setting myself up for heartache but i also know he has noone in his life, no parents or close family so what shall i do? listen to my heart or my head