If you are only going for SA using 'normal' training then I'd stop with this guy. He is what is called a 'slow' trainer, not too common but there are enough out there to be annoying.
I train up 4 or 5 18yo's every season to 6* then just let the game keep them at that for about 3 seasons then they slow down. Every couple of seasons I'll get a donkey like yours that trains about 60% of the others. I'd keep him, forget about the SA and just let him gain naturally and sell at the end of the season, but maybe think about a replacement. Chances of getting 2 slow trainers in a row are pretty slim.