In connection with meeting the Financial Requirement it can become complicated.

Unless your partner is in Category A 'Salaried Employment' where his monthly payslip is based on a minimum 1/12th of annual salary of £18600 or more then he would need to show income over a 12 month period.

Based on what you've shared, he's working on a variable income 'Non-Salaried' employment where one or more of his monthly payslips may have fallen below the requirement.

In this case he should review being assessed under Category B for variable income.
Meeting the financial requirement under this section means having income evidenced in two parts.

(1) The annualised gross annual salary or income from salaried or non-salaried employment at the date of application. (Total gross income from all employment undertaken during the 6 month period, divided by 6 and multiplied by 12 = Income from non-salaried employment that can be counted towards the financial requirement.)

(2) The actual amount of gross income received from any salaried or non-salaried employment in the 12 months prior to the application.

I'm not sure how UKVI would view applications for unmarried partner given an absence of 12 months or more.