After giving him the empty xp bottle and buying a few xp potions you'll unlock the black xp potion. Then if you have a creatine and a black xp potion in your inventory you should have the option to combine them.
If you have both of those items and it's not showing up, then I'm not sure. The only thing I can think of is if you got the black xp potion through the VIP item menu then it might not show up until you go through the process of giving him the empty bottle