I would reccommend getting his Immunoglobulin levels tested, he could have Iga Deficiency or another immune disorder.
Does he have any stomach symptoms as well like loose stools, cramps etc?
I was a sickly child and it turned out I had Coeliac Disease which is a gluten Intolerance that actually damages the lining of your intestines and then you don't absorb nutrients properly and get recurrent colds and flu's all the time, but there are usually some sort of stomach symptoms with it as well.
I also have Iga Deficiency, and the two quite often go together so it's worth getting him tested for both.
Vitamins with iron, iron rich foods, vitamin c, lots of orange juice and consider an allergy specialist. This makes me think of my childhood. It's very lonely when you miss alot of school. I wish him well.
Have him checked for Epstein Barr Virus. Its similar to mononucleosis, however its symptoms come and go. Its not too sever,(most cases) and usually can be treated with proper rest and fluids and TLC. My son got it when he started school, he was run down all the time, always getting sick, turns out his gmom had it too! dont know who gave it to who but it is passed through saliva, body fluids, etc. And kids share water fountains, boogies, all that great fun stuff. Its worth a shot and hope he feels better!! It does require doctor to give prescription for bloodtest with EBV on it.
One good thing to try are vitamins. It's important the child gets a full spectrum of vitamins to maintain proper energy levels. It's really hard not having a history to diagnose anything, but it could be anything from depression causing the lack of energy, to diet, or even a compromised immune system. That's just my personal opinion.