Reoccurring illness

My 4.5 year old son has had such a bad run of flus this winter (yay for spring!!) and i'm so sick of him being unwell. There have been times when he's needed antibiotics and others he has not but it seems that ever 2-3 weeks something else will being. It usually involves a lot of green snot (sorry for the grossness) and he's missed so many days of Kinder and FDC.

I know it's a given to some extent that he's going to get these things now that he's out in the big wide world so much more but is the extent of it called for? My daughter will sometimes be ill at the same time as him but not always and then my eldest is so good he only had his first day off school for the year last week.

What can I do to help him? I'm going to the Dr's today, i'm thinking he may need a multi-vitamin maybe? His diet is great on the fruit side of things but not great on the veg/meat side of things.

Another concern I have is he has his favourite blankie that he's extremely attached to and unfortunately he sucks it. I wash it when I remember and when I can get it way from him but is it possible that this isn't great for him? Can these things carry germs as a result of him sucking on it? I would love to wean him off it but it's not going to be easy (a whole other thread really).

If you have any ideas of how I could avoid so much illness for him please let me know.