Hi
generally speaking if the case was based on the document which was alleged to be forged , then the sec.420 does not have nay base.
However needed to see the FIR to know why charge under 420 IPC has been retained while U/s 471 IPC dropped.
what was the step taken in the court . did he filed a petition to drop charges and court dropped 471 and found him guilty under 420?.
If so his option is to defend it with all available means of evidence to prove his innocence, it will be favoring him in the trial as 471 is dropped .
if the document submitted was genuine then the intention of submitting it knowing that it was not a genuine one is difficult to prove.
If the sufficient evince has been found to maintain the charge under 420 , the same can be tried while other charges are dropped due to lack of evidence.