I swear that on the many occasions that my IDP was demanded at checkpoints it's very presence was all that mattered. I don't think that the date on the cover, your picture or your motorcycle endorsement is comprehended. Only once did an officer open the thing to take a quick glance at my pic. Of course, all this WOULD matter to your insurer if things go sideways.
He sounds too stubborn and green to pull off anything subtle or sophisticated. I can't quite get why he's ignored about 40 reasonable replies suggesting he pay the fine and leave. Something we don't know?
Your health insurance coverage could be denied if you had an accident and didn't have a motorcycle license for your home country. Or maybe not, but I don't like that bet.
I always get an IDP in Canada. Over 8 winters I've had to produce it around 20 times at Thai checkpoints. Only once did the cop actually riff through it and check my photograph. Even he didn't stop to see if I had the motorcycle endorsement stamp, which I do have and better have or health insurance will be voided for any accident. Most guys immediately lost interest when they saw the grey cover of the permit and waved us on. One point is that the western date makes as little sense to them as Thai dates make to me. They don't even try to suss that out. Sometimes, I think I should have just used the same one for 8 years and saved a few bucks. Another point is attitude. We didn't get 500 meters from the bike rental in Chiang Mai and got stopped. The permit was buried under the seat and as I searched I could tell he was bracing himself for a story. "Oh dear, must have left it at the hotel" or some such thing. When I finally produced it he broke out in the greatest smile, like I'd just won a lottery. Just another reason I love Thailand.
You're good then. We've done 8 multi month winter trips to Thailand and clocked over 20,000 km on scooters. I always get one of these, CA$25 from CAA, our auto club. On average, three stops at checkpoints where they look for helmets and permits. (Typically a fine of Baht 600 for the absence of either, paid on the spot and then you just drive on.) Almost always, the mere sight of that grey cover elicits a smile a wave or "Where are you from/going?" Only once, did he actually open the folder to look at my passport pic inside the back page. What became clear over time is that they really don't want to read it. Thais everywhere have a commendable command of basic English but reading, not so much. Add in the fact that their Year is about 500 years different than ours and that the motorcycle endorsement stamp isn't that easily understood anyway and they're just not going to waste their time. I resisted just keeping the 1 year permit for 8 years, which would work, since insurers might be more attentive than the cops if we got in real trouble. You're going to LOVE Thai touring.
Even local mail is very slow in Canada right now. My past experience with Thai to Canada surface packages has been 4 months transit. OTOH, filing deadline postponed to June 30 and payments due back to August 31. I filed a simple return on the web with CRA recommended free software and it was painless and got my small refund in less than 3 weeks.