There can be many causes of low testosterone in a 13 year old boy, but if it were my son, I would do a full blood panel on him to determine what the levels of "competing" hormones might be, such as estrogenic levels, how healthy his pituitary gland is (functioning of human growth hormone - how is the growth of this young man, his skin condition, emotional condition, etc...). As young men grow through puberty, the complex homeostatic condition of his body changes, which affects a myriad of hormones in his endocrine system. I would not be alarmed if his spirits, activity levels, appetite and other cues are normal. If he is complaining of lethargy, headaches, dry mouth and other symptoms, the blood panel will likely reveal what may be affecting his testosterone levels. Certainly, once you get a clearer sense of the symptoms, changes in habits, activity levels, certain foods (like excessive simple carbohydrates being consumed which can affect hormone levels), then you can better focus in on what underlying issues may be causing this reduction in testosterone. Please, however, be reassured, that "low" is quite subjective for a 13 year old whos body is going through substantial changes. Keep an eye on him and short of specific conditions, you might consider a change in activity (weight bearing activities which product more LH from the pituitary that sends a message to his testes to produce more testosterone) or food, with a focus on lower phytoestrogenic items like soy,wild yam products and testosterone precursors (foods and supplements that help spur on the production of testosterone without using the actual hormone patch, injection or other delivery system).