Hi there.
If your mother is not married to, nor lives with her current partner then we do not need to look at his income. If your mother simply puts that she is 'separated' or 'single' whatever her situation may be, then we can look at your mother's income only.
We look at 'household income' so if your mother's income is the only income within your own household then that is all we require.
If he does in fact live with you permanently then we would need to look at this because as mentioned above, we look at the income within the household as a whole.
Thanks.