Or use a Quickdraw Shield with the Quickdraw Feat.
That would get you AC after casting the spell, not during like the OP asked. Seriously though, get a wand of shield (and UMD if necessary).
Ah, it depends on how you understand his/her use of 'attacking/casting' - during the actual action (eg vs AoOs) or simply during the rest of the round.
Overall I agree with you: a Wand of Shield (or a custom magic item with uses per day if Shield isn't on your spell list - if you can get it, it can be better than investing in UMD as a fighter) is the way to go.
Like Gilarius and Ozy said, a wand of Shield will be a better choice, or just him casting Shield, since as a magus it is on his spell list. More AC (well, until you are at the point of a +4 or higher magic buckler), no penalty if he wants to two-hand his weapon on rounds he isn't doing spell combat, works against incorporeal, and even if its a caster level 1 wand a minute is more than enough for most fights. Plus if he casts it himself he can cast it during spell combat, so he doesn't even have to give up a round of attacks (there is an arcana that would allow him to use the wand during spell combat, but probably not worth it unless he starts using other wands as well).