If your milk tastes odd it may be an idea to exclude pregnancy as a possible reason. Though taste change could be as simple as diet changes.
Could he be refusing to feed as he is now managing to go longer between feeds and take more and therefore simply isn't hungry?
I found that during partial nursing strikes lots of skin to skin is brilliant. I never needed to express as I could usually get LO to feed by waiting until nap time, whipping my top off and lying down with her. Once she got tired she was always willing to feed, even if she had been screaming at the boob a few minutes before.