Weakened immune systems accompanied by infections

According to the data you reported (swollen lymph nodes in the neck, armpits, and groin are; low blood count, low glucose level, fatigue and lack of appetite) your brother might be experiencing infection accompanied with weak immune system.
The weak immune system might be caused by repetitive and heavy infections that exhaust the body, including the immune system. If the repetitive infections are not so heavy, then the immune response gets stronger and the body tends to fight them in earlier stages.
However, the presence of enlarged lymph nodes might be found in some blood disorders. In most cases, like lymphoma or leukemia the white blood count is actually raised, instead of lowered as in your brother. There is a possibility that the bone marrow, which produces particular parts of the blood, is also affected, thus causing the low blood count. Please note, that there are many various types and subtypes of lymphoma and leukemia, and they have different treatment protocols and differ in prognosis.
In the end, giving such diagnoses like lymphoma or leukemia over internet might be overdiagnosed. A specialist in hematoloy might help you. You would have to wait the results from the biopsy to determine the exact cause for the enlarged lymph nodes, which as previously mentioned might have trivial reason.