Boomerang son - just keeps coming back

My wife and I have a son that is involved in a problematic marriage. Every few months for the past several years (he's been married about 10 years) his wife decides to kick him out. He comes and lives with us a few weeks or months and then she asks him to come back. At times on our exhortation he will go to counseling a few sessions and then quits. Both have been in counseling together but only short-term. My wife and I are in our mid-60s. She thinks we should continue allowing this but I feel that this cannot continue. Can you help?