I am not able to update whatsapp as it keeps asking me to enter password of my old apple id.

I am constantly asked for my old iCloud password on my whatsapp to update n I don't no the password it's my sons old account

That means that the app was downloaded by his account - if you want it linked to your account delete it and download it on your account

By 'old account' do you mean that you are now using a new account, or that you've updated the primary email address on the account and you are being asked for the old version's password ?

If the former then you should be asked for the password of the account that you used to download the app - all content from the store is tied to the account that downloaded it.

If you can't remember the account's password then you can get it reset : http://iforgot.apple.com

If you've updated the primary email address then try tapping on your at the bottom of the Featured tab in the App Store app and log out and then log back in with the updated version of the account