This could just be skin infestation by lice or scabies. scabies usually is accompanied by bumps on penis and severe itching during night. Do u have any bumps on penis..? Sometimes internal disease like liver disease,kidney disease, diabetes, thyroids disorders and blood disorders are also associated with itching without any visible signs. As you said it started right after your friend visit it probably might be lice or scabies. But for any itching without any visible rash detail systemic examination should be done to exclude internal disease. please take a second opinion from the doctor if itching is severe without any rash. if everything comes out normal than taking antihistamines ( anti-itch medications) and slowly decreasing over 3-4 months might help.