Dear Sir,
"UAE Company does not need any relieving letter of present employer" - If UAE company do not want this document, please ignore about getting relieving certificate and go-ahead with joining. However...
An organisation can change their internal policies any time. In your son's case if the amended rule has come into effect after he putting his resignation (though you vaguely mentioned), then the old clause appearing in his appointment letter will be considered (arguably even that email posted date appears after he tendering his resignation can also come for his rescue). Because many things can cook up in the organisation's kitchen, unless it is served in time its validity will not come into force.
Ask him to preserve copy of resignation letter and the email circulation.
Secondly ask him to update his new employer about this relieving related issue. And the above docs will help him to substantiate his current situation, if the new employer demands it.
Third thing if he can convince his current employer, take their confidence up, keeping a good relationship, request them to consider by allowing him to payback the notice period, for a cordial relationship. Certain companies will allow this depends on the relationship.
All the best for his future endeavors.
Suresh