My son has a bump or lump on the back of his head (occipital bone) it seems to come and go away. It does not hurt when he touches it. Should I be worr?

Simple vs serious. Our bodies always have things that grow and change. Skin, fat, muscle, lymph nodes, bones, can change shape, get larger, more prominent due to many factors. The vast majority are not cancer but simply unusual growths like fatty tumors, cysts, skin tags, moles, etc. Most cause no symptoms and can be watched. If they cause symptoms or change colors, bleed, bruise or hurt, then see your doctor.
Lump on Back of Head. A lump that comes and goes is likely to be a lymph node. Children with particular viral infections, scalp infection often have enlarged occipital lymph nodes. Any lump that persists, becomes larger or is tender is best checked out by your child's pediatrician.