I’ve found these stay on very well at night

Clear Passage Nasal Strips Extra... https://www.amazon.com/dp/B075JKBVCZ?ref=ppx_pop_mob_ap_share

A few things:

My partner found [these earplugs](Ear Plugs for Sleeping, Acousdea Reusable Moldable Silicone Ear Plugs, Waterproof, Suitable for Snoring, Swimming, Working, Studying, Noise Cancelling up to 40 dBSPL, Mix with Carry Case, 6 Pairs https://www.amazon.com/dp/B08SWGKSFC/ref=cm_sw_r_cp_api_glt_fabc_31KH2AMDTD6CZZAT36E5?_encoding=UTF8&psc=1) to be very comfortable versus standard plug ones

I use [these nose strips](Clear Passage Nasal Strips Extra... https://www.amazon.com/dp/B075JKBVCZ?ref=ppx_pop_mob_ap_share) and they help

Have him download the Snore Lab app and run it for a few nights (while you sleep in another room 😁) so he can see the data and hear it himself.

Next step would be to see a sleep doctor or sleep apnea specialist.