19 month old son has marble sized lump behind ear and two smaller ones on back of head all feel fixed and quite hard no other symptoms but im worried?

Lymph node. There are many lymph nodes in the neck and back of the head that are often more noticeable because they are located where our fat pads don't hide them. If you feel around you should find several the size of a pea ; sometimes bigger.They can increase to the size of a big peanut if stimulated by local factors, illness or vaccines.I would just monitor it or have doc see it during regular visit.