Sounds like limit reached.
- You are not willing/able to change to monoshipping. You already know that does not work for you. Your setting is (polyamorous and polyshipping)
- He has tried polyshipping for x time and finds it does not work for him. He has found is he not (monoamorous and polyshipping) in his preferred way of going. But (monoamorous and monoshipping) in his preferences for loving and relating.
You both love each other. But things do not line up so it can coexist well at this time. Basically could ask him if he is willing and able to continue... Or willing but not able? And having to digest that. Or you could decide to make the call if you see he is not able.
Perhaps the most loving thing you can do right now is accepting limit reached and becoming willing to release what does not work -- the relationship model? And perhaps considering a relationship model that could work for both -- friendship?