If you already have a recent version of Microsoft Office, it may include a free trial of Publisher that you aren't aware is right there in your Office package. Did you look, if you have Office? There are free trials of certain programs included when you buy Office.
If not, did you go to the Microsoft web site (not some other site, Microsoft's official web site)? I would check there, and search on the web site for "Publisher free trial."
Is this edited assignment due tomorrow? If so, I would let your son just deal with the consequences if you cannot find a trial of Publisher that works tonight. I would also make him responsible for telling the teacher that you do not have Publisher at home and he (your son) will have to do any further edits at school. This is a chance for him to learn not to assume that he has access to the same tools at home that he has at school. (And for the teachers to learn the same thing if they somehow assume that every family has every program that's in use at school.)
Why do you say "I am unable to do so"? Are you doing the editing or is he? This is his homework -- I hope you are not planning to edit this yourself in any way, Publisher or not.