1. Since your son is not interested in obtaining divorce, let him file a suit for the Restitution of Conjugal Rights.
2. After filling for Restitution of Conjugal Rights and obtaining favourable order for RCR, still if your daughter-in-law does not join your son in their marital life, then her refusal to join your son in marital life, inspite of the Court order, will be a strong ground for your son to obtain divorce, without paying any alimony/maintenance amount.
3. Without proving the allegations, the case may likely to be dismissed.