Depending on how much the leg is bleeding, i'd try to stem it, either by using a dressing, clothing, or a make shift tourniquet. Whilst he's on his back, have a quick look for any other big bleeds. We're talking about pools of blood forming. The blood loss could kill him.
If he's unconscious and not breathing, then that could be because of a blocked airway, which you cant tell with his lid on. The helmet needs to come off.people are extremely wary of c spine injury and are totally against taking a helmet off. If theres two of you, you can remove it carefully enough to protect the spine. If not, then it needs t be removed. If you leave it on, you'll take a corpse to hospital, safe in the knowledge that you didn't damage his spine. When you remove the helmet have a good look for damage too. To roll him over, try to position his body into the normal anatomical line, ie straight, with hands by sides. With you kneeling by his hips, roll him over, trying to control his head- quite a challenge with only two hands!
once the helmet is off, check his airway is clear, if it isn't, try to clear if, but only as far as you can see- don't stick your fingers down his throat... To maintain the airway, you could perform a head tilt, or a chin lift/jaw thrust manoeuvre, but the jaw thrust needs to be manned constantly. No good if you're alone.
if he isn't breathing (a 30 second check) then commence cpr. 30 compressions, to 2 breaths, at a rate of 80-120 compressions a minute, roughly one third the depth of his chest.
Another thing to check is whether or not the leg is still bleeding. He could have stopped breathing due to cardiac arrest, caused by hypovolaemia (low circulating volume). Cpr needs to continue until an ambulance arrives and takes over. They'll be giving him fluids, some cardiac drugs possibly, and defibrillation. Bottom line is that he need defib, but your good quality CPR is allowing the brain in particular to be perfused and remain alive, long enou for the ambulance to get there.
there are differing opinions on when to phone for help. Id probably give 2 breaths and 30 compressions before phoning. You can always pause the call periodically to continue cpr- the operator isn't going anywhere. Failing that, talk clearly and fast before hanging up- they can replay the call to get the info.
if a fire crew arrives first, dont neccessarily expect them to take over from you. If it looks like youre doing a good job, they may leave you to it, unless you ask for help!
i think posts like this are very useful. I regularly run through "what ifs" and I think it better prepares you for the day something actually happens.
hope this helps