What Causes Burning And Tingling Sensation At The Tip Of Penis?

My boyfriend has a burning tingling sensation in the tip of his penis/urethra. He states that it only burns before and after urination, sitting, and laying down. He states that the sensation intensifies when laying down on his right side and is the same normal sensation when laying down on his left side. I examined his penis and he shows no signs of a red irritation, discharge, or bleeding. He also shows no signs of blood in his urine so I was unsure if it could be a kidney stone. At first I thought it could actually be a kidney stone because of the constant burning and tingling sensation and because it takes him a little longer than usual to urinate. I also asked him if he had taken a shower before work this morning and he said yes. Normally he has to rush his showers and was thinking that he might have gotten soap in the tip of his penis/urethra. Please help me with some answers and let me know if he needs to take a trip to see a doctor. Thank you for your time and patience. -Kristen