My 7yo has had enlarged l. Neck lymph node for past 5 yrs. Doc & hematologist say its nothing. R. One was big for 1yr but went down. Why l. Still big?

See below. Lymph nodes are 'houses' for white blood cells (in addition to filtering debris and germs out of the body). Sometimes, just as when you build an addition on your house, you don't tear it down when you don't need the space anymore, the lymph node stays large. As long as it doesn't keep growing and doesn't get red, tender, squishy, or rock-hard, it can be monitored by the doctor at checkups.
Infection ? Enlarged lymph node are usually in responsed to infections such as tonsillitis tuberculosis virus etc if not more than 2 cm in size no need to biopsy it will go away.