What Causes Low Stamina And Tiredness In Children?

my grandson is 11 years old. He has always been very active, plays sports, particularly travel soccer. About 3-4 weeks ago, he had a virus for 2-3 days. He recovered from it but has lost stamina and tires easily. The layoff from practice may have resulted in him being in not good condition. He is still tired a lot at school and lacks stamina still. Additionally,he has been going to bed later than he usually does. 10 pm instead of 8 pm. He shows no other overt symptoms, so I m inclined to rule out mono. We are taking him to the doctor tomorrow. Do you think blood work is in order to make a diagnosis?