Without examination,confirmation of a diagnosis is tough but as all STD’s have been ruled out,so it can be due to urethritis or contact dermatitis. Urethritis (an inflammation of the urethra) is usually obvious because of a urethra discharge and there is often burning with urination.
A contact dermatitis means that the penis is coming in contact with something it is allergic to. The common products would be bathing soap, detergent used for laundering, industrial products on the hands that are not washed thoroughly before urinating, feminine hygiene substances your partner might use, sexual lubricants, condom preservatives, etc.
Pls use a mild soap like dove and mild detergent for laubdry washing.Also use cotton undergarments and wash your hands before voiding.If the symptoms persist then it would be better to consult a dermatologist.
Hope it helps. Take care and pls do keep me posted on how you are doing or if you have any additional queries.