Student or Learner
Are both correct? We are acting on his behalf and We are acting in his behalf.
I think you'll find that our AE friends will tell you that 'in his behalf' is just fine, too.
Not me, I'd only use "on."
***** NOT A TEACHER *****
Here is a "rule" that some people follow for more exact communication. I offer it with the gentle suggestion that you follow it.
In behalf of
For the benefit of or as a champion or friend.
"A large sum was raised by the women's society in behalf of ailing [name of nationality] children."
On behalf of
The agent or in place of
"The lawyer entered a not guilty plea on behalf of the defendant."
Complete credit goes to Dos, Don'ts & Maybes of English Usage by Theodore M. Bernstein with the assistance of Marylea Meyersohn and Bertram Lippman (New York City: Times Books, 1977), p. 28.
"Now that you have seen the distinction exhibited in examples, doesn't it seem natural and worthwhile?" asks Mr. Bernstein, who worked for years at The New York Times.