What Causes Swollen Lymph Nodes In A Toddler?

My 2 yr old son had bronchiolitis when he was 7 months old. Then he developed slightly swollen lymoh nodes on the sides of his neck. So he was asked to go for blood test. His report read that his eosnophillia count is 5. He was under medication for 15 days with Banocide. Even then the lymph node didnt disappear after 8 months. Again the doctor took the blood test and mantoux test. He cleared the mantoux test but esonophillia was 4%. Again he was prescribed with medicine for eosnophillia and worm infestion. Now one of the nodes disappeared but one still remains. What would be the reason for swollen nodes and test is need to be taken?