i wouldn't get him immunised if he is not feeling well - mainly for the fact that he is just not himself.

they say the immunisation is for between 4 and 5 yrs old, i know with my son i got his done at the age of 4yrs and 7mths.

maybe wait a week or two especially with christmas coming, you don't want him to be feeling miserable with a virus and also immunisation discomfort when santa arrives.

hope he is better soon