Only Witches, Wizards and Arcane Blooded Sorcerers can get Familiars.
Then perhaps the boon text should be revised to make this more clear. As it stands, it says "casters." Otherwise, nongurus waste a lot of time trying to make it work for the wrong type of caster.
I would add that there may be other extenuating circumstances where it may not work. Despite the instructions, it seems that it may be more complex in some cases than simply adding the Improved Familiar feat, even for the correct type of caster.
Even if a draconic sorcerer can't have a "normal" familiar, having access to one that possesses some draconic angle seems to make sense. Oh well. The final word seems to be No.
My son was overjoyed at the idea of gaining the familiar for his sorcerer. Couldn't wait to add it, since he was moving up to L7. However, the number of prerequisites makes it nearly impossible to add this after the fact. We tried through Hero Lab and the app said no. A draconic sorcerer needs to have a load of prerequisites to make this possible, and even then it's toying with the system. In fact, ANY sorcerer is going to have problems utilizing this boon. So the "caster" prerequisite is hopeful at best. Oh well. This is the sort of minutia problem that dogs Pathfinder, in general, and makes it harder for nongurus to do things well. Just sayin'.