i feel sorry for your son but he need to have more tests run and blood work.
Dehydration can cause a stroke and siezure in a normal eighteen year old. In the military there are documented deaths from over-training. Dehydration can be deadly, and often the most motivated people fall victim. Clearly he needs a proper medical evaluiation, which is absolutely positively impossible on this forum. Yes you should be concerned, but don't panic. At the age of eighteen he will be incredibly rsilient. Pale skin is an indication of inadquate perufsion and there are many simple reasons for this to occur. Getting a hematocrit would be a good start (red blood cell count). Nose bleeds are curious animals. Sometimes we never know the cause. Of course a good physician will run various simple blood clotting tests to determine if there is a problem there. Good nutrition and adequate hydration are very important. The numbness is an issue, but I have insiufficient information as to even hazard a guess as to the etiology. Get yourself a good caring M.D. who will pay attention to our son. They are hard to find these days, but they are around. As to the sequence of events in terms of the siezure and the head hitting the ground sometimes it is impossible to see whether the chicken came before the egg. Did the siezure cause the fall, or the lack of oxygen cause dizziness and a fall and an impact which disrupted the brain and caused the siezure. A single siezure episode does not doom one to a lifetime of anti-siezure medications. Everyone is capable of an epileptic siezure. Some pilots experience a siezure when they go into the FAA hypobaric chamber, for example. Others ocasionally sieze when experiencing flickering lights. Helicopter pilots in fine physical condition have gone into siezures from the flashing of the sun between the rotor blades. So don't be overly concerned. You have to keep close tabs on things and will have a period of what is called "watchful waiting". The chances are he will live a healthy and fulfilling life.