I need to get my sons bail reduced. Can I send a letter to the judge on his behalf? He is 19

Usually a bail reduction request is made by the attorney at a preliminary hearing.  You can write the letter, but it is a good idea to copy his attorney on it, the prosecuting attorney on it, and then the court.  Otherwise, the judge would likely ignore it, which he or she may still do.  I would give the letter to his defense attorney to seek a bond reduction.