Because he was earning above the required amount from the same employer before his raise, will 6 months of bank statements and payslips suffice?
I ask because I am checking "no" in both the 3.12 and 3.13 boxes in Appendix 2.
Since he has been earning more than £18,600 for the whole period, he can apply under either Category A and send 6 months, or under Category B and send 12 months.
If he sends 6 months of bank statements and payslips, then UKVI will only take the LOWEST of the 6 payslips and use that to calculate the salary used for the application. As long as this payslip is still more than £1,550 before tax, giving at least £18,600 when multiplied by 12 months, he will qualify under Category A.
However, doing it this way may affect how you answer the questions on Appendix 2... because you may need to use his OLD salary for the questions, not his new one, in order to be able to answer YES to 3.12 and NO to 3.13 instead of the other way around.
So, basically, you have 4 options for how you apply:
1) Tick Category A, use his OLD salary for 3.11 and then tick YES to 3.12 and NO to 3.13... they will use the lowest payslip to calculate the income, which should therefore match the OLD salary you put.
2) Tick Category A, use his NEW salary for 3.11 but still tick YES to 3.12 and NO to 3.13 (since he has been with the company more than 6 months and has been earning over £18,600 for the whole time)... they will use the lowest payslip to calculate the income, but this will end up being lower than the NEW salary used in 3.11
3) Tick Category B, put his NEW salary for 3.11, tick NO to 3.12 and Yes to 3.13, then include 12 months of payslips and bank statements
4) Wait another month to apply, so that you have 6 payslips showing the new salary and apply under Category A as normal.
Really, though, it doesn't matter which boxes you tick because he meets the requirements anyway, regardless of which category and salary you use... it's just going to be a case of deciding which way you feel most comfortable doing it.