Thanks to all for your thoughtful replies.
I managed to speak to him properly and we got somewhere, even though it got quite stressful.
He said I don't realise sometimes how much it affects him when I go through stressful times (just to give context, over the last 11 months I've had to deal with the death of a friend, the stresses of buying, renovating and then moving into a flat on my own, various financial pressures and going freelance in my work. Apart from the bereavement these are all positive changes, but very very draining). He said during this time I've turned from someone who is normally pretty chilled to being much more controlling of our time together, creating routines (when we eat; when we go to bed, etc.) which have made him feel stifled and treated like a child. So he's found it increasingly difficult to be spontaneous or explore with me due to being under pressure to behave in a certain way around me. Neither of us take well to feeling controlled.
He accepts that he tends to engage in avoidant behaviour. In his mind he was being considerate, because I was stressed and he was being patient thinking things will simply sort themselves out once I'm settled again and we can play again. He said he thought his avoidance was a good thing because he was able to behave in the way I wanted, instead of turning this into a confrontation.
I explained to him that maybe he meant well but in fact, what he did was changing his behaviour instead
of talking to me - and then this resulted in him not feeling able to express himself fully with me.
I'm the kind of person who would always rather have difficult or painful conversations , so I at least have a chance to change things. Avoidance doesn't work for me, and unexplained changes in partner's sexual behaviour towards me really really mess with my head. So now I have to undo months of feeling rejected/frustrated and all the resulting messy feelings, whereas knowing this ages ago would've been harsh to hear but given me a choice to avoid this situation.
Anyway, he's agreed to try to be less avoidant and I'm going to try to be more relaxed. I've asked to see each other less for a few weeks so I can flush toxic negative thoughts out of my system. So far it seems to be working...
In the meanwhile, does anyone have advice on how to deal with the interim period when I know he's having this kind of sex with her and not with me (until things go back to normal sexually for us, which I'm now more confident they will)? I also know that exploring kink is a big part of his NRE behaviour and at core he isn't really that kinky, so this won't be the last time I'll have to deal with this kind of jealousy/envy. I know I also get to play with others but I would never do something with another person I'm not giving my primary. A counter argument is to think about how supportive he's been with me over the last year and that we all get different things at different times... still, perspective is helpful. Thanks