i'd call everyone and let them make the decision. is he well enough to enjoy the party? will the outdoors ie wind/rain/heat/sun make him feel worse? Also, do you think he's contagious?
people might not be happy if he's sick and they take sickness home, particularly if they have a new bub or a frail or elderly person to see/care for iykwim so i think it best to be upfront and let people know.
maybe you can postpone? poor little darling, being sick at your party would suck!