Can anyone please help, our 6 month old pup, toy breed, has always done the normal humping anything that moves, we just gently move him off the object or person and try to distract him, we have no problems with this behaviour.
Our real concerns are when he gets over excited he starts rolling around yelping, crying and getting very distressed, he can't walk properly when he gets up, this can go on for some time with him eventually being able to walk around, but looking very sad, tail between the legs and really unhappy.
I am trying to describe everything without being to explicit, I don't want to offend anyone by sounding like I am being rude, we have seen the vet who was very sympathetic when I explained everything, but gave a little laugh when I told him that our puppy often looks like he has five legs as his private bits are always on show, and he was shocked to see all the bruising that this problem causes.
The vet gave our pup an injection, a form of chemical castration.
This is very distressing as the puppy often seems in so much pain and we can't do anything for him, we all get very upset especially the kids who just can't understand whats wrong.
Can anyone give me any advice?
Is this normal behaviour?
We have had dogs before but have never seen anything like this!

How awful for you (and the dog of course!) Do you mean that when he has an erection his penis does not return into the sheath? If so a simple snip from the vet may help. I know it's graphic but you need to spell it out for us and the vet to understand the exact nature of the problem!

I'm guessing your pup may have a condition called 'paraphimosis'. This is where the opening of the prepuce is unusually narrow and his penis cannot retract properly following an erection. As you've seen, this is very painful for him, and can be dangerous. It is important, when his penis is exposed, not to let it dry out, or it will become even more swollen and painful. If dipping it in a bowl of cool water, or smearing it with vaseline, doesn't help it back, surgery will be required to increase the size of the opening.

If I've misunderstood the problem (your post wasn't very clear! ;) ) I apologise!
:)