There's nothing definitive....
to say that he is the imposter.
Although they allude to and leave open the possibility that he's NOT Sommersby, I still want to believe.
They beat us over the head with the long story about how the best thing that happened to Horace was that he looked like Jack Summersby, but I think it comes down to a choice for the viewer.
Personally, I want it to have been Jack Summersby all along. I want to believe a man change change and become the ideal that he strives for.
When I reflect back on this movie (and it's really underrated (that's just an appraisal), I will always believe from start to finish, we only see Jack Summersby on screen.
It just makes me happier that way.