For an public-school, Oxbridge gent who's worn pips on his shoulder, ending up as a clerk, salesman or the like would have certainly been a 'step down'. Ideas:
1/ General manager of some small firm. His old CO, a relative etc hears of the MC's predicament - considers it 'a scandal' what his father's done, 'specially as the MC's a veteran. However, they've got 'an interest' in firm X, and they pull a couple of strings to get him hired.
2/ As a 'London connection'. Say his old CO, cousin or a schoolfriend went off to the colonies and made a tasty little business out there (Plantation? Mining? Whatever) But they need a 'man in Blighty' to handle the work on that end. They remember the MC as being 'a decent fellow' and offer him the job.
3/ Stockbroker. If the MC's got a few friends in the City, he might consider playing the markets - if he's got some cash to invest, that is. While he might have been cut off by his father, chances are that some other relatives might still chip in, leave him some cash in wills, and the pile of back-pay he'd earned in the War. In this period the markets were much more sedate (in the UK, at least) - and insider dealing wasn't illegal at this time.
4/ Starts his own company. During the War, the MC gets facinated with some 'new' technology. Cars, Areoplanes, Radio etc. He teams up with some engineer (perhaps an NCO who'd served under him in the War) and goes into business. The NCO handles the technial stuff, while the MC manages the thing. Perhaps he sinks what cash he's got into the thing?
EDIT: Upper-middle class at this time liked to copy the upper class at this time - it was best to have your own 'country estate' and live off rent. Failing that, investments. Failing that, 'managing' something - though better to manage something on your own account than for another (someone non-family, that is).
Chances are, he wouldn't go into the civil service - they got real bloated in the War, and were in the process of being slimmed down. 'Direct sales?' Too vulgar. He wouldn't have any of the technical skills to go into the Professions (solicitor, accountant etc). Low-status job like a clerk, librarian etc? Might as well as become a navvy. (in the eyes of his peers at the time)