My son is 2years and 6 month old. I have noticed lymph nodes behind his ears and neck when he was 4 month old and they are still there. Is this norml/?

Yes. Yes, this is typical. Babies and children get many viral infections during this time. Their immune systems are constantly adjusting and responding to new viral offenders. Antibodies are created in these lymph nodes and they "react" to new infections and become noticeable in the neck area. If you see a node that is fixed, red, hot, or tender, then this needs an evaluation.