Do you breast feed him for long? Coz I strongly believe that breastfed babies immunity is very much stronger then non breast feed babies.
I breastfed my 1st daughter for bearlya month and she too fall sick quite frequently during her infant year until she's 2 yrs old then I start giving her vitamins like Colostrum, Vit C.
While I breastfed my 2nd daughter for 7 mths (she's 8mths old now)
she only fall sick once and is just mild runny nose and cough which recover within 3 days. Even now if my elder girl is sick, my baby so far is strong enuff that she never get e virus from her sister.
Yr baby is only 4mths so you can't give him anything to boost his immune until his around 1year + or 2yrs old. If you suspect yr baby got it from yr elder child then you shld try to seperate them away shld either of them get sick. Let them sleep on different room, etc.