My son Ben had that when he was 9. All he wanted to do was lay down, he was so tired.
His liver also got infected (hepatitis) and our doc was really worried.
They let him stay home with meds, but he was really listless and we were scared.
It took about a month before he was back to his old self.
Dell will get better. Little kids have a lot more ability to heal than us older folks do.
Just give him lots of hugs and comfort, and popscicles.