Pay him - you may expect too much - when papering a room with a chimney breast (or even without) I have never achieved an exact match at the end point - the technique is usually to try to achieve a good match all around and leave the "error" in the least noticeable and darkest corner.
The problem is caused by the plastering - just tiny variations in depth of plaster around the room all add together to make exact matching very difficult if not impossible. Worse in an old house where the lime plaster finish may be up to 20mm thick.
I have had missmatch by up to an inch - but in a dark corner with all the furniture put back and the curtains up - it never gets noticed unless specifically looked for.
If you're dissatisfied with the overall quality of the paperhanging (gaps and mismatch all over) then negotiate a lower price - it would be highly unethical for you to refuse to pay anything - the fact that he is not as good a tradesman as you hoped - is because you selected him (probably on a lower quote) it's a buyer beware situation.
Good luck...
Peccavi
September 2010