I´m not going to butt my head in too far and ask you why it is so important that he take it sublingually. Don´t get me wrong, I respect that being able to stick to longer-onset ROAś can be an important stepping stone in their recovery. However, the whole thing is basically inconsequential. You need to have trust in your relationship. I know that´s not what you want to hear. If you have to wonder, you should be direct with your partner and ask.
The intranasal bioavailability is commonly stated at 50% whereas the sublingual bioavailability is ~30%. If he is insufflating his medication, then it is quite close to taking a double dose at any given time. This is not typically dangerous, as Buprenorphine has a great safety profile. It´s not going to kill him, however Buprenorphine is a drug for whom the pharmacology changes drastically as the dosage is raised above 2mg per day. Essentially, the Buprenorphine just using my own understanding and rationale of this drug, is at its best ultimately below this dose.
I´ve known people who have gone from SL, to insufflation to injection chasing a high that they all ultimately admit they never even got. Everyone that I have ever personally spoken to has stated in retrospect that it was their conditioning to know Opioids as a cure for how they feel that they continue pushing the dose upward - and doctors are more than happy to facilitate.
If you need more information, let me know. Just ask him. If you saw orange crumbs in his nose, don´t be a bitch, you know that no normal person walks around with orange crumbs in their nose for any reason, don´t take any bullshit or any deception. It will only hurt you both if you are afraid to confront them. I´ve lived it, I´ve done it. I´ve told utterly ridiculous lies to women I´ve loved tantamount to having orange crumbs stuck in my nose.
Good luck. Stand your ground.