I was just looking into this for a client last week.
Unfortunately. The 475(f) mark-to-market election is required to be filed with the timely return (or timely extension request) for the prior year return. So the earliest he could elect mark-to-market is for the tax year 2020. For 2020, the election deadline is April 15, 2020 with his return or extension request.
If he was unaware of the election until now, taxpayer may be able to elect for 2019 by requesting and paying for a private letter ruling under 9100-3, however this is not automatic and can be denied by the IRS. Not sure if 9100-3 relief would work for prior years. His practitioner probably didn't tell him about the election until now (one of the qualifications for relief), but he would save a lot of tax based on hindsight (which would probably prevent relief from being granted).
Without the mark-to-market election, he may still be a Sec. 162 trade or business. This would allow you to avoid NIIT (since, as a day trader, his business activity is nonpassive), and deduct his margin interest and other expenses on Schedule C rather than Schedule A. Without the 475(f) election, he'd still report trades on Schedule D. HOWEVER, some court decisions have held that the mark-to-market election is one of the indicators of being a trade or business so there's a risk of going for those deductions.
The guidelines for being a Trade or Business of securities trading [url="https://www.irs.gov/taxtopics/tc429"]include[/url]: regular and continuous (more than a year); attempting to profit by market movements rather than investment or dividends (short-term trading); frequency and dollar amounts of trades; time devoted to activity; using the activity as a livelihood. I'm not aware of a 4 transactions/day guideline.
Some courts have held that having a full-time job apart from trading is an indicator that gambling or trading is not a Sec. 162 business, using language like looking for the activity to be taxpayers "sole or primary income". However it seems that those cases, the taxpayer also wasn't really trading super regularly. So a regular, substantial trader with another job might qualify but there is risk of being disallowed.
At any rate, it seems he is no longer "in the trade or business" and can't take the election next year.