My son went on camp for 5 days to Brits, on returning he went for a pool party. He is now complaining of warmth inside his body and particularly on the back. He has a runny nose and his eyes are red and watery. When I felt him he was not hot.

Allergies. I believe you are describing allergic rhinitis (hay fever) having camped outside and exposed to pollens, etc.
He can take safely Claritin 10 moor OTC Loraditine
pills 1x/d as indicated for symptoms. He can instill
OTC Vasocon A 2 drops eyes twice a day for eye redness and itchiness. Welcome to health tap, Dr. Ivy Fisher.