I believe you can exchange it at RBI
Go to them with proper proof of withdrawl of your 50k. It's your money in the end and 50k is a good amount- hell any amount needs to be changed.
Many people have forgot their money or found it stashed away.
give it a try
I think it is black money that is the only reason I could think up for him afraid to go to bank and get it changed to new notes.
Op you are not required to disclose anything because 50k is well within limit.