He is scared of the stairs themselves? or is he scared of what is upstairs? Is he physically able to?
Have you tried using high value treats such as chicken, hot dogs, cheese, freeze dried liver treats to lure him up the stairs? You don't have to go all the way in one day. Go at his pace
. If he will go up one step on day 1 on his own terms, work your way to two steps, and so on. You need to associate the stairs or being upstairs with positive things, so treats and praise, a favorite toy, a favorite game to play with you, etc. Counter conditioning. Basically you are going to change his emotions of fear of the stairs (assuming he is scared of going upstairs or the upstairs) with a sense of pleasure/calmness/happiness (he gets treats and praise and toys!)