My son is 5 and ever since he was little he has gotten bronchittis and pnemonia atleast once every year since hes been born what could cause this?

Immune deficiency. Recurrent serious infections in a young child is suspicious for some type of inherited immune deficiency. You might consider bringing this up with your pediatrician, or asking for a referral to a specialist to investigate further.
Common in some. This pattern is not uncommon in kids who have RSV bronchiolitis their first winter, or allergy spectrum issues like eczema, toddlers asthma ( which most out grow by ~5). If these issues were handled at home with oral meds the process is likely transitional. If these events required inpatient care i would wonder if a selective immune issue was involved.