If it's a newish relationship and you've not been used to calling anyone 'Sir' before it might be something which will come naturally in time. I find it a little odd you're being punished for it and it might be worth halting it until your relationship is a little older and the words might flow more naturally.

I know it's not the same, but I'm an extremely forgetful person and it's not something I can easily change. I've learned over the last two years to improve it and I forget less things but it's not because he threatened punishments if I forgot.

Give it a little time, and go easy on yourself, saying Sir naturally doesn't come easy to everyone, if at all.