My son is seven months old and teething, was struggling with fever 38.4 - 40.4c for three days but that has cleared up now he has a rash on his tummy and behind his ears paired with a swollen lymph gland behing his left ear. what could be the cause?

An illness: I don't accept the idea that this was from teeth. This is a common pattern for many viruses that fever on/off for a few days ending with a rash as the fever leaves. The lymph glands often swell with such illnesses & the most evident are those in the neck/head where they are not hidden deep in the tissues.

Evaluation : Please have an evaluation by your pediatrician. He may hav an ear infection, pneumonia or Strep. Watch for eye redness, peeling skin and edema. Treat fever with Tylenol rather than Motrin. Consider COVID screening. Often a viral illness is the underlying cause.