Im sure everyone is wondering how axel got a keyblade, including myself.....so i came up with 2 possible theories on why axel can use one now:
1) Axel gained a keyblade because of his time spent with roxas...maybe a part of roxas is with axel or something like that......i remember from the kingdom hearts 2, axel said roxas made him feel like he had a heart.
2) this one is far out there but, in the scene in KH 3D where xehnort is trying to transfer a piece of his heart to sora, axel ends up getting in the way of his attempt. so maybe that piece of xehnort is with axel now instead of sora.
These theories are kind of dumb but let me know what you guys think anyway because it cause it is still a major mindf*#! for me.