Hi and welcome to the forum! I am glad that I can help you.
You would like to know whether increased white blood cells may indicate an underlying problem.
Usage of amphetamines among all the other effects may cause anorexia and weight loss. Due to the anorexia there can appear a decrease in the number of all the blood cells, not only of the white cells. After stopping the intake of this drug there can be withdrawal symptoms, but if everything goes well the body should return back to normal. Increased white cells may indicate infection or leukemia.
Given the data that you described, you husband may or may not be experience any medical condition. He should consider seeing an infectologist if he experiences increased body temperature, fatigue, sore throat, cough, back pain, or any other symptom that may suggest infection. He may need to have a bone marrow biopsy to rule out leukemia. He may also consider having blood tests for common tumor markers to rule out cancer.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact his doctor or other qualified healthcare provider such as an infectologist or a hematologist for further advice and information about diagnosis and treatment option for your husband's current condition.