It does become a problem that a combat familiar isn't able to face enemies without magic fang or DR. So it makes sense that a Hexblade would want a little more mileage out of their familiars which are meant to do battle. Having to rely on potions for something so basic does seem out of character, since they cannot cast Ghostly Visage or Stoneskin.
To me this looks like a classic example of a balance boulder based on the most powerful class crushing the rest.
I think I'd like to see a spell like that one introduced. I doubt it would be a noticeable buff to wizards, who have such an immense toolkit already; it may give them another thematic option but that is all. For Hexblades, it would make the "summoner" playstyle attractive, especially in groups. I also think the spell could easily be 1 hour + 1 turn/level without being overpowered, though its ability bonuses might need to be reduced; in general, familiars have very poor combat stats already, but the fighty ones can be buffed up to about the strength of a normal character if you really need it.
I think you will find it would be entirely fair if the spell granted DR like so: 5/+1 at level 4, 5/+2 at level 8, 5/+3 at level 12, 5/+4 at level 16, 5/+5 at level 20.
Pure hexblades' familiars would be able to land hits through Stoneskin, and eventually Premonition, for a little bit of extra damage that is hardly game-breaking but could be useful as ambient damage while the familiar tanks what it can.