I'm worried because my son is blood group is o-ve blood group. The doctor said, that this blood group is rare, what do you suggest?

Minor issue. People with rh- blood grouping live full and uneventful lives all the time. Blood banks love to see them come around because their type can be donated to anyone, but they can only get blood from a compatible rh- donor. Most blood banks and hospitals keep donor units in stock.I'd quit obsessing and move on to issues you can change.