Most of these suggestions are geared around hitting the character despite his huge investment in AC enhancements. To me, the most likely result is a bigger arms race - you can still hit me reliably, so I need more AC enhancing abilities.
We're seeing only one character. Do the other PC's have equal wealth? Have they designed less optimized characters? IOW, does this character stand out as significantly more powerful than the rest of the PC's and, if so, why? Maybe the other PC's need a step up to match him (typically better received than taking something away to level the field). Maybe no adjustment is needed at all - are the players enjoying the game? If so, is there really a problem?
We're seeing only one character. Do the other PC's have equal wealth? Have they designed less optimized characters? IOW, does this character stand out as significantly more powerful than the rest of the PC's and, if so, why? Maybe the other PC's need a step up to match him (typically better received than taking something away to level the field). Maybe no adjustment is needed at all - are the players enjoying the game? If so, is there really a problem?