Your son can file for divorce on grounds of mental cruelty
2) refusal to have sex amounts to mental cruelty
3) in the event nay settlement is arrived at petition can be converted into divorce by mutual consent
4) your son ought to have filed for anullment of marriage within period of one year on grounds of impotency of wife
5) your son can file cheating case against his wife but then she would file counter case of dowry harassment / DV against your son