My son, who is 18, caught acute mono over 5 months ago. He had a severe case and was unable to attend the last 3 months of school, and was one of the worst cases his doctor has seen. The severe part of his mono has gone away, but since then he still struggles daily. He has little or no appetite, strength, and seems to be constantly not feeling well. We've taken him in to get bloodwork and the results have come back 'normal'. My question is: Is this common? If not, what should we do?

The older the patient, the more severely they are affected by Infectious Mononucleosis.
A period of post-mono with the symptoms you describe is common. This may last as long as 6 months although 3 is more usual. This is despite the blood work becoming normal.
There is little you can do.You might try tempting him with his favourite foods.
David