Son's 2nd molars are in. dr saw holes/pits (not cavities) on buccal side and recommend it to be filled? is it necessary since there is no decay?

Fill the holes: Yes, even though there may not be any decay right now those "buccal pits" will be spot where bacteria loves to hang out and cavities will develop over time. It is much better to place a small filling in them now to avoid a large one later.

Necessary?: How is necessary defined? Anyway, pits have a high tendency to decay early treatment is preferable for long term life of the tooth and filling. Less invasive the better. If your child has a history of dental caries and the pit is suspicious a fill is in order. If he has no caries or few a sealant restoration may be sufficient.

Reactive v Proactive: If you seal those pits and fissures with a sealant now, you will help prevent cavities later on. Once they decay, the will need to be drilled and a filling will need to placed. Sealants are conservative and help preserve tooth structure. It is money well spent.