Try keep an eye on what food he eat and start to analyse if it happen after certain food.

How is his water intake?
My boy was a very good water drinker, he drank large amount of water daily. Therefore he never sick other than got a chicken pox at age 2.

Besides that, Scott fish oil is good for kids. It maintain good immune system.

He only 2yrs+, right? Other than baby food, or those brand from the shelves that specially mean for babies, don't give him other snacks. I recall I never give my boy sweet/candy or even jam until he was in kindergarten. And we never give fruit juice, but eat real food in small chunks, as fruit juice sugar level higher.