I think that you are getting really good advice. maybe try asking how he is feeling if you notice a change? Or if you see him glazing over stop and tell him that, he may not realize right away that he is doing it. And then give him a chance to come back or talk about it whatever he needs. I still struggle at times with being present during sex myself and sometimes wont even realize I'm not present unless my partner calls me on it.
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Everything comes from within