passenger seat belt ticket

 This  afternoon I got stop by city police  and  I wonder why  ( May be I did not stop at stop sign at local road or I change line without light ) It is almost near my home and my son is on back passenger seat and busy for his cellphone ( he is a very busy for home work and college application).
When The police came near by. I asked my off the car to go home for homework. I wanted to deal with the police only. I thought is  my fault but the police said my son did not wear seat belt. I was supervised. On that time  My son already off the seal belt. He said at first time "he is  wearing the seal belt"  when the police asker again  he said  he was not sure because he already took off the seal belt, then he will be silent,

   I checked  the city web said  $146 fine over 16 year old and  $449 under 16 year for drive or passenger seat belt ticket , plus court fee It will be  a big money. I am going to court  tomorrow  to see how much  the total the fine is  
 
   My question is how the police show the evidence that my son did not wear seat belt on the court ( from back it is hard to see the  seal belt|. From right side  ( I sure the police did not  drive my right road , left side is curb)but police said  he was on my right side) If it possible my son was busy on his cell phone (低頭看手機) so  the  police did not see clearly

    My car is Toyota camry  2012 which has  seat belt alarm and  We didnot hear any noise. If near by end of Sep  may be the police has number ticket to go?

    Do I have a case if we go to court to appeal ?  I am in southern California  ( I had experience won case on the court 10 years ago in NV)

Thanks in advance  for suggestions