@kazu19 There's a maximum possible potential for a lifetime. Even though he is working on all of his attributes now, he still has a lower maximum possible potential in Red magic then he does in any other magic.
From Chapter 1:
As you can see when he went back, his max stats didn't change. Red still maxes out at 62 while Green maxes out at 99 and spirit maxes out at 97.
In other words even if he dedicates himself to red magic YET AGAIN, he will still hit his cap at 62 which was the entire reason he was booted from the position in the first place. He could instead work on becoming the best Green mage while also working on Red on the side and get recognized as the best Green, but he isn't for some unknown reason. Heck he could even do it with Spirit if he wanted to.
*edit* I did a quick scan of the series to check his stats,
chapter 5:
chapter 14
chapter 17
chapter 18
chapter 28
chapter 40
So yeah, his max hasn't improved at all. No reason to dedicate to becoming the strongest red mage at all.