He's always ill

My 2yo is always ill! I know people say it's normal for this age but he's always been like this and he gets everything....way more illnesses than his peers. This year he's had tonsillitis 4 times needing antibiotics, chickenpox, gasterinteritus, chest infection, UTI, rotavirus and umpteen colds/viral infections. This has resulted in lots of time off preschool (he's only been going since August) and knocked his development too. 

He's had bloods done to check his immune system and check him for a variety of other things but all came back fine and within normal ranges. He did have a high B12 result but nothing they appear concerned about.

Anyone else have/had similar and can offer some advice to increase his immune?