Once it is established he has epi he will be given a enzyme for his lacking ability to digest fats and absorb certain vitamins. This enzyme must be taken with every meal and probably for the rest of his life unless some temporary and fixable condition can be found. He will gain weight once he starts absorbing nutrients from his enzyme digested food. They might also make sure he doesn't have a B12 deficiency which can be controlled with B12 injections. Work closely with your Doctor and you will get there!🙂