Legal Question in Criminal Law in California

You can ask, but it won't have any effect. The time to discuss sentencing with the court was at the sentencing hearing, and in papers filed in connection with that hearing.

No. First, the judge cannot consider any letter sent directly to them - they must notify both attorneys (prosecution and defense) and then calendar a hearing if necessary. A letter sent to the judge will do nothing.

Secondly, the sentence has already been imposed. Could he (through his attorney) seek a modification and/or to withdraw his plea for some reason? Maybe, but that is a discussion that is left to him and his attorney.

Bottom line - no, your letter won't accomplish anything and may only make things worse. Don't do anything on his behalf, trying to "help" him unless it's cleared through his attorney first.

You can waste your time and write; the judge can't and won't read it, and he can't and won't take any action on your sympathy plea, even if he wanted to. The case is over.