Hello, Tina. I'm still learning too, and so I hope you receive other insights from the OPing experts.
I brush before OPing, and then swish out my mouth with a cup of warm water with 1/2 tsp of Sea Salt ... I'll take a swig and swish & spit, and take another swig and swish & spit, and take another swig and swish & spit ... then gargle with it ... and finally, I'll brush again. (I hope that's alright?)
I wonder if it'd help your BF to switch oils? Maybe the one he is using is too potent for him right now?