from what I understand from the Bible, you are only allowed to divorce in case of cheating

Greeting Craig, so from what I researched Jesus gives instruction on this in similarity to what Martin says:
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(Matthew 5:31) It hath been said, Whosoever shall put away his wife, let him give her a writing of divorcement:
(Matthew 5:32) But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced committeth adultery.
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Fornication in verse 32 has the Greek translation of πορνεία [ porneia – G4202 Strong’s Concordance ]

“illicit sexual intercourse
adultery, fornication, homosexuality, lesbianism, intercourse with animals etc.
sexual intercourse with close relatives; Lev. 18
sexual intercourse with a divorced man or woman; Mk. 10:11-12

metaph. the worship of idols
of the defilement of idolatry, as incurred by eating the sacrifices offered to idols”
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It would be most logical to conclude that illicit sexual intercourse such as adultery, fornication, bestiality, homosexuality, incest, etc. would be a valid grounds for divorce. I do recommend that your son communicate with his wife and try to work things out under Godly council. Women tend to act on how they feel in the moment, perhaps get him to start taking her out on dates more like they did when they first met. Complacency can kill any relationship.