Because handwriting identification needs your client's handwriting, without it, handwriting identification can't be completed, and according to your situation, you may need to sue before applying for handwriting identification.

But a word of caution, if the customer intentionally writes different fonts when signing, then you may just be wasting your time. You can ask him to sign in the form of debt maturity confirmation, and see if he will not notice the signature for a while and form a new debt confirmation.