He may not come upstairs. On the other hand, he may already be coming upstairs, unbeknownst to you, when all is quiet - when everyone is out or asleep. My mother has a very shy cat who mostly lives in the basement, in hiding, but comes out at night and ventures upstairs. The evidence is seen in the clumps of long white fur from where she has lain down. Sometimes my mother just sits on the stairs to the basement, and Magic comes partway up and stands on the stairs to be scratched. You could try that as an enticement.