Botulism... there have been how many cases in the last 30 years in NZ? 1? He would need to eat a fair bit to get it.
*I* think small amount of honey is ok for an 11 m/o, do you have a vaporiser? I have been using baby balsam (by Vicks) for good snot nose relief. As for the cough just keep him upright when he is awake as much as you can, lower his milk consumption, offer more water and maybe find some sugary goodness as sugar is a great pain killer for kidlets
I hope he feels better soon!