Use this tool as a guide, for you...to understand why he's paying that $4,000/month
https://www.justice.gc.ca/eng/fl-df/child-enfant/2017/look-rech.asp
Does $4,000/month seems like a reasonable amount to you?
How much net income does he have per month after that $4,000 is spent?
Based on the information provided, I see that he has one option:
I'm not saying that he's in the wrong, but based on information left out it's hard to decipher. I don't know if he has 50/50 custody or if he has very limited visitation rights. Reason for the split, reason for the custody dispute and financial stats of both him and his ex wife before/during and after the split.
It's a tough pill to swallow. I don't think he'll find a lawyer to do pro-bono or provide council on "contingency". Best course is to lower his standard of living and save, save, save to be able to pay for that lawyer.