Igor Pak gave good advice on what to write in your paper. Additionally, I would advise you to contact the author of the original paper and tell them about your work before it is published. This can be a good opportunity to get to know someone who shares common interests with you and it opens possibility for future joint work. I'll bet if you talk to the other author the two of you together are going to have a new idea which is even better than his or yours alone.
And another thing. If you write to the author and start talking to him, you will have a chance to find out about the background of his work and you won't have to imagine useless things (such as "this other guy is so stupid"). Perhaps he knows that his theorem sometimes reduces to a tautology and will readily agree with you. Perhaps he thinks it only happens occasionally and will be grateful to you for explaining that this is not so. In any case, just talk to the other guy.
