Well, I would double check with CMS, this will help:
<span style=”font-size: 11.0pt; line-height: 115%; font-family: ‘Calibri’,’sans-serif’; mso-ascii-theme-font: minor-latin; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-font-family: ‘Times New Roman’; mso-bidi-theme-font: minor-bidi; mso-ansi-language: EN-GB; mso-fareast-language: EN-US; mso-bidi-language: AR-SA;”>Child maintenance calculator: https://www.gov.uk/calculate-your-child-maintenance</span>
Meanwhile it depends what these other financial commitments are that you took out together – if they’re in both your names (such as a loan) then he is legally culpable. If not, to what else are you referring?