I am not an expert, and anything I say here must be considered an outside opinion at best, but what I don't understand is - you say your man earned 21k last year. He is also self employed.
Well presumably he has an accountant to handle his finances. 21k is well above the income requirement.

Could he not then, get his accountant to draw up the necessary proofs of his income, withdraw the current application - yes it means losing the best part of £900, this isn't something to be taken lightly. Then put in a new application giving this financial requirement meeting evidence - as this is the only thing the previous one failed under. I would however suggest he finds a decent solicitor to go over the application to check it's watertight against the current rules (As it involves self employment, this isn't as straightforward I'm guessing as an employed person able to produce wageslips and employers letters). This of course would be another couple of hundred pounds for the solicitor, ish.

Yes this proves a bit pricier, but otherwise, this appeal is going to string out over a couple of years with no guarantee it would be overturned, in fact it could well go up.

To pay for that, he could cancel one of his planned trips over which would easily cover the cost. He won't like that of course, but think of it this way - cancel one trip and in return in 2 months you might have the ability to be together and make future trips not necessary.

IF of course I'm not overlooking something obvious.

But like I said, I could be way off the mark here