For the most part, there is very little risk to drinking lots of water. Our kidney's are designed to maintain the blood at a certain salt concentration. If someone drinks a lot of water, the kidneys will just make urine with more water than salt. Thus, drinking more water than you need is not helpful, but also generally not harmful. The only danger that can arise if the water your son is drinking is consumed too quickly for his kidney's to deal with. If this were to occur, then the salt concentrations in his blood would drop. This can be very dangerous and even life threatening. The good news is that this is very rare. We see it occasionally when someone consumes a lot of water (more than a gallon) in a short amount of time as the result of a dare or a contest.
Since your son is embarking on a routine of a lot of exercise and changes in his diet, I suggest that he schedule an appointment with his primary care physician
. He or she can review his exercise routine and diet and suggest any changes that would improve his health and reduce his risks of any future health problems.