Here is an idea, based on what you said:
The Magus' unique ability is to ignore Concentration. The down side is that like the Warlock, he only has a tiny number of spell slots, a tiny number of Spells Known, and his spell list is very short. I would have him gain spells at the same rate as a warlock and have the same upper limit on spell level. Early on, it's just nice that he can cast Bane and not worry about getting hit. At high level, he can cast Greater Invisibility, Haste, and Fly, but that would still take all his spell slots..
You can add invocation-like things as well to add various passive effects (at-will Mage Armor, an extra weapon attack, a single cast of Fireball each day, that sort of thing).