What could we use as back-up to prove his secondary income?
If you are convinced it is necessary, go to section 9 and check 'Other' and in the box, write "Sources of Income". Then attach the statement that lists the sources of income that he has. Remember that they are only supposed to look at the next 6 months anyway.
As to your second question - about what monthly income is appropriate, I don't have enough information about your app to make an informed decision. They consider the region, the prospects, your resources, the cash-flow. I always try to tie it to the ONS, but that may not work for you. I had given a link to their most recent report in one of the threads below seeking advice about putting a budget together.
