Looks like a side effect: I would take him into you pediatrician first to make sure it is not pneumonia first... Antibiotic might be needed. Hope he feels better.
Seek medical attention right away if any of these SEVERE side effects occur when using Sulfasalazine:
Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); bloody diarrhea; bluish discoloration of the skin or nails; chest pain; dark urine; decreased urination; fever, chills, or sore throat; hearing loss; mental or mood changes; muscle pain; numbness or tingling in the fingers or toes; pale stools; persistent loss of appetite; pinpoint bruises; red, swollen, peeling, or blistered skin; seizures; severe or persistent dizziness, drowsiness, headache, or trouble sleeping; severe or persistent stomach pain; :heart:shortness of breath;:heart: trouble walking; unusual bruising or bleeding; unusual tiredness or weakness; unusually pale skin; yellowing of the eyes; yellowing of the skin along with dark urine, pale stools, or persistent loss of appetite.