It's sounds to be multi-factoral, and needs to be examined, have blood work, by the doctors for proper diagnosis. I can relate to all his symptoms, but may not be the same his, as are my causes. If he is only 3 months post treatment, it's still early, and may actually be doing well, even to have attempted working. Sometimes it can take a full year to recover. It's taking me much longer.

GERD
CIF...chemo induced fatigue
Anemia
Malnutrition
Dehydration
Low blood pressure

Good luck with the doctor tomorrow.

10/09 T1N2bM0 Tonsil
11/09 Taxo Cisp 5-FU, 6 Months Hosp
01/11 35 IMRT 70Gy 7 Wks
06/11 30 HBO
08/11 RND PNI
06/12 SND PNI LVI
08/12 RND Pec Flap IORT 12 Gy
10/12 25 IMRT 50Gy 6 Wks Taxo Erbitux
10/13 SND
10/13 TBO/Angiograph
10/13 RND Carotid Remove IORT 10Gy PNI
12/13 25 Protons 50Gy 6 Wks Carbo
11/14 All Teeth Extract 30 HBO
03/15 Sequestromy Buccal Flap ORN
09/16 Mandibulectomy Fib Flap Sternotomy
04/17 Regraft hypergranulation Donor Site
06/17 Heart Attack Stent
02/19 Finally Cancer Free Took 10 yrs