What insurance was your partner under while he was on his IEC visa? It's my understanding that he could have been on OHIP three months after he arrived with his original OWP with 6 months of full-time employment secured (ie, a letter from an employer).
I'm in BC for context but my situation was this -
When I first moved to BC with my IEC visa, I thought I couldn't get MSP at all. By the time I realized I could, I only had 5 months left on my IEC visa (you have to have valid temporary worker status for six months to get MSP, and then you have to wait 3 months after that) so I was stuck paying private insurance until I got a second IEC visa. Then I applied for MSP as soon as I got a new visa, and 3 months later, got an MSP card in the mail.
It sounds similar (albeit, Ontario requires you to have full-time employment, BC only requires your permit be good for 6+ months and then you have to wait 3 months from the time you apply) to Ontario, even if it's not exactly the same.
The Ontario website doesn't say anything about a 3 month wait, which is strange, so maybe it is different than what BC requires.
Edit: To clarify, no, the time he was working under an old visa would not count - that's how it was for me. They don't care that I was working in Canada before, as I didn't have MSP at that time so I was a "new" resident by their standards.