these have helped me, a chin strap is cheaper in the long run. But, this will not necessarily seal your mouth shut, it may help, or it may not make any difference at all. A full face mask may be more appropriate for you, based on the information that you provided.
I use these. I will say I tear it off in my sleep almost every night. I’ve only been trying it for a few weeks tho. It is a little addicting as I generally wake up feeling more rested.
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No I actually haven’t noticed any adhesive smell/taste. I use these
There are some Amazon tape things that he can use. Lol sounds weird but the the mouth stays close so people breath through Their nose. Sleep Strips 60 Pcs,Advanced... https://www.amazon.com/dp/B07X27FV15?ref=ppx_pop_mob_ap_share