Welcome to the forum!
I really encourage you to try supplemental enteral nutrition (EN). Fatigue was definitely one of my son's symptoms prior to diagnosis but, even with continued inflammation (but no obvious symptoms), fatigue has only bothered him once in a while since diagnosis. (My son is also very involved in sports.) His maintenance treatment since July 2011 was only supplemental EN. The EN formula provides him with 1500 calories (and most of the necessary nutrition) per day, 5 days per week. (He very recently began remicade as there continued to be inflammation but will continue with EN as well.)
I do believe that the EN has kept his nutritional needs at a good level and this may have helped fight off fatigue and other symptoms. In my son's case, he ingests the formula overnight through an NG tube and eats a regular diet during the day. However, there are various shakes that can be taken orally, ie Peptamin, Boost, Ensure.
Many kids on this forum use supplemental EN together with their maintenance meds.
However, I do believe methotrexate can cause some fatigue as a side effect. I don't have experience with mtx but I
believe folate is taken with mtx (not at the same time!) and it may be to help reduce the fatigue??? But, I'm not sure...
Is he on weekly shots? Is it possible to change the schedule of the day of his shot? Perhaps if he is most fatigued following the shot, a change in day would alleviate some of the impact on his lifestyle.
I'm sorry it's your son's illness that brought you here but glad you found this forum. Lots of great members and parents with tons of advice!
Good luck!!