As mean as this sounds... The best way to deal with said issue is to let it happen. What would be the harm in letting him tank one boss one week, then let the warrior tank it next week. Use WWS reports and show him the difference. He seems to theorycraft alot, yes? Use facts against him. It may be the easy way out that you are looking for.
In all honesty though, I dont think your paladin is the only one causing the issue. There isnt alot of harm in letting other people tank. I was forced as the MT of my guild because I am the only warrior. We also have two prot paladins and a feral druid. There are plenty of fights where a warrior just is not the ideal tank for the encounter. Sure, we can do it, but does that make it the most efficient? It did not take long for my guild to accept that fact that when I refused to tank a boss, there was a reason for it. Our first Azgalor kill will be tanked by a druid and our first Gorefiend kill was tanked by a paladin. Its just safer.
You said yourself that you and the MT have a very good relationship, but thats not enough to be honest in a tanking squad. You need to find everyone's strengths and weaknesses and utilize them effectively. In all honesty, it doesnt hurt to ...as they say... throw the dog a bone once in a while. What is the harm of letting the paladin tank farm bosses if it makes him happy? How else will he get better if he doesnt get the practice? You can only learn so much from tanking trash and 5mans.
Though, I did somewhat stray from the topic at hand. Your best bet is to just let him tank one boss one week, then let the warrior tank it the next. Use wws parsings on both of them and show to him the difference. My guess is, you'll find the paladin was safer, but the warrior made the boss die faster from sheer threat alone. That may be enough to shush him up, or who knows, your MT may find a new strength in his tanking regime that he refused to see before. Being the MT in a guild isnt just about tanking all the bosses, its about making the smartest decisions when it comes to tanking the bosses...and sometimes sitting yourself is that very decision.