You did not include how much your son is eating so I am going onthe premise that his eating habits is on par with the rest of his age group.
He may have a couple of things going here some good and some bad.
He may very well have a high metabolism which is burning up the food as fast as he is getting it in. Thus his body does not have the energy /
materials it's needs to grow and build itself.
He may also have an inability to process certain food items such as dairy products. This would produce the weakness as well as some of the other
symptoms that you descibed such as the fingernails and toenails.
The worst possibility is something like Rickets
Which also fits many / most of the symptoms that you have described. I would suggest that you take your son to your pediatrician, explain your
concerns and have the doctor perform some tests to identify what (if anything) may be wrong with your son.
Please let me know how it goes. I have some friends who suffer from Rickets so I am familiar with the deficency