Dear Mrs Ervin
Thank you for writing Justanswer with your concern.
I've been reviewing several sources, sorry for the delay.
The only thing that seems to fit with his profile is an atypical fungal infection. There are many types of fungal infections, and not all of them involve thickening or yellowing of the nails. Especially in a young, clean, and otherwise healthy individual. With his sports activity and the description you have given (some toes with the discoloration starting at the tips I really think this is likely.
Otherwise for "blue to grey" or "brown" nails other problems that can cause this would be silver poisoining (for grey or blue nails) or mercury poisoning (for brown nails) But if he had this degree of poisoning, he would certainly most likely have other symptoms at this point.
One of the best treatments for nail fungus, that does not treat the whole body (such as oral medications) is tea tree oil. This comes usually in a small glass bottle and you can buy it in any pharmacy. This oil is very concentrated and strong. It will actually soak through the nail to the underlying skin, which is needed to treat toe or fingernail infections. This can be applied directly to the nail, and after the first few nights, only a tiny bit will be required each night to each toe. You could also and soak the toes in a solution of warm water with several drops of the tea tree oil. Only take care not to get this in the eyes.
I have seen this (tea tree oil) work quite well, but according to the Mayo clinic there have not been enough scientific studies to yet prove the effectiveness in all cases.
See this article from the Mayo Clinic, regarding tea tree oil if you like:
This is the Mayo Clinic's recommendation for treatment of nail fungus, that is based on empirical evidence. Description of toenail fungus is given as well, causes, treatment and prevention recommendations.
If you are interested in alternative methods, this article by Andrew Weil, MD, a physician who studies complementary or alternative medicine, may be of interest to you. One of his recommendations is Tea tree oil. http://www.drweil.com/drw/u/id/QAA362690
With any medication for nail fungus, you will note that results will only be visible as the nail grows out, so, look for the new nail growing in to be clear, as well as there should be no increase in discoloration elsewhere if treatment is working. It will also be important to perhaps have an extra pair of clean, cotton socks to change into when he is away from home and is changing shoes to run for example. Washing the feet carefully daily and after sports with soap and water, and drying very thouroughly will help prevent further infection with nail fungus or other fungus.
If he does not see improvement within a few weeks of beginning the treatment that he chooses, or other symptoms appear, please see a physician for physical examination and work up.
I hope this is helpful. If you do have other questions or comments, please reply so that I or another expert can address them.