[Cocoa's Momma]

This is rather embarrassing but, Cocoa is 4 months old and he has been humping things a lot, including my leg...he was doing that tonight and I picked him up to say NO! and he had a full erection and it was very dark red and he was breathing really heavy...it took a few minutes to go back down. What does this mean?? Is it normal for his age? Should he be neutered? I appreciate all the help I can get!