I have no idea how reliable the site below is, but it has an enormous list of possibilities for "slurred speech" and to my surprise, many of the most common ones are allergy or insect bite related - which might make sense given the sneezing. How soon are his appts?
Btw there is some scary stuff further down the list but remember that as you go down the list the likelihood goes down.
http://www.freemd.com/s1/slurring-speech.htm