Should I inform my thesis adviser about current situation
As a rule of thumb, if you tell someone you'll be doing stuff with them at time X, and then change your mind, let them know!

More specifically, I would recommend an email such as:

"Dear Advisor,

we had planned for me to start my thesis in September, but because of REASONS I'll only be able to start on October. I apologize bla bla
In the meantime, I'm planning to do EXERCISES. Please let me know if you have any other suggestions on how to prepare."

I don't see a point in telling this to your advisor in person anyway, and this should keep you safe from being made to sign stuff.