Dog won’t go upstairs?!

Ok weird one for you... We have a one year old Jackapoo.. we're convinced he came from a Puppy farm but that's a whole other story. Anyway, he won't go up the stairs in the house. We've tried everything but he just won't, he's terrified bless him! We've tried treats, pulling him (gently) on his lead but nothing works! The weird thing is, if we're our on a walk and come across steps he's absolutely fine and doesn't think anything of bounding up the steps over bridges etc. it's literally just the stairs at home he won't deal with... Any ideas?!