Opinions vary widely, so I expect this thread to soon turn into a stealth "house rules" thread. But the core rules are:
(A) The cohort possibly makes a slight change to the average party level.
(B) NPC's, including the cohort, get a half share of experience.
Also, the DMG lists a "simplified" way to figure a "half share" which is not actually a half share and which, in my opinion, fails to be simple. That way is this: figure the NPC's share as if he were going to get as much as anybody else. Then give him only half of that and divide the other half up among everybody else.
This will case the cohort to slip behind the rest of the party in experience level, which many people have a problem with, but not as much as you might think.