mono symptoms but negative test

My teen son got what appeared to be the flu in February (per a pediatrician who did not test him for flu). After 2 weeks he seemed to get better only to get sick again the next week. This time another dr in the group said he had classic mono symptoms (he even thought he could feel his spleen tip--note that my son is normally pretty thin). But the test came back negative.
After weeks of fever, sore throat, EXTREME fatigue, malaise, dizziness, etc., we got re-tested--again, no mono. Dr. did a CBC, a CReactive Protein test, another mono spot, and thyroid function test. They also did a chest xray to look for hidden swollen lymph nodes (he said he couldn't feel anything swollen).
My son missed 5 weeks of school (he's a straight A student) and couldn't participate in his sport and he became very depressed during this time. When we went back 2 months later about the fatigue and the doctor said it was probably psychological. We've seen a psychologist now for about 6 weeks with no change (he says the only thing he's sad about is how tired he is!).
He's unable to get back to his previous performance levels no matter how hard he tries (it's been more than 3 months after the mystery illness). He's on summer break now (managed to pull all A's) but he's still very upset about how TIRED he is. He also has had a fever (102 degrees), dizziness and nausea 3 different times since then that leaves him completely wiped out for several days afterward.
DOES ANYONE KNOW IF YOU CAN HAVE MONO YET HAVE NEGATIVE TEST RESULTS? Or should we be looking for something else?