It's not that he can't use his magic at all, more like his mana level is really low due to Ross's Heartguard Dragon Scale that was implanted into him when he got his heart pierced by a traitor in his team. The dragon scale requires a huge amount of mana to function properly which is why all the mana that Leon had was drained away by the it, preventing him from unleashing his full potential. He does have a backup plan which enables him to use his magic.
WN Vol 3 Ch 108:
"By the way", Leon said, "my inability to gather magic before...is that connected to the dragon scale?"
Rossiver nodded. "After the scale was implanted, it needed a huge amount of magical energy to function properly."
"With your constitution back then, there's no way you should have been in a coma for two whole years, even with the blood feud's side effects."
"But when I implanted the dragon scale, it drained all of your magic, leaving your body in an incredibly weakened state. Combined with the blood feud's impact, it's what kept you unconscious for so long."
Leon nodded thoughtfully. "And now, after all these years, I still can't store magic on my own?"
"Unfortunately no. For a dragon to form a Heartguard Dragon Scale takes at least two hundred years and during that time, we have to nourish it continuously with magic. It's only been five years for you, so...you're about hundred and fifty years short."