Alternative idiom to "phone it in"

He's really just going through the motions.

From the [Free Dictionary]:

go through the motions

Fig. to make a feeble effort to do something; to do something insincerely or in cursory fashion. Jane isn't doing her best. She's just going through the motions. Bill was supposed to be raking the yard, but he was just going through the motions.

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

go through the motions

Do something perfunctorily, or merely pretend to do it. For example, The team is so far behind that they're just going through the motions, or She didn't really grieve at his death; she just went through the motions. [c. 1800]

The American Heritage® Dictionary of Idioms by Christine Ammer.

go through the motions

To do something in a mechanical manner indicative of a lack of interest or involvement.

American Heritage® Dictionary of the English Language, Fifth Edition.

He's making a token effort.

http://i.word.com/idictionary/token

sense 2(a)

The above also mentions perfunctory. You could write that he did a perfunctory job, or that he did the task perfunctorily (don't try that last one in speech—you'll stumble on the pronunciation.)

How about "Half-assing" something?

"I don't feel like working on this essay. I'll play video games and just half-ass it later."

Not an idiom, but the person you're describing is performing a Cursory effort:

rapidly and often superficially performed or produced