It is normal but....definitely try to keep him from "humping" things...I know Mojo use to do this..even at 3 and 4 years old....He rubbed a raw spot on his wee-wee and it bleed for so long...it was horrible. The vet said that if it gets "stuck" out (Mojo's would hang to the floor)
That it can cause serious damage! We had him nuetered last year and haven't had a problem since!