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Did they say why his application was declined?
Unfortunately, it is a little known fact that many countries refuse to grant visa's to persons with criminal records. Some countries are more strict than others, and some are more concerned with some types of criminal offences than others. Unfortunately, countries are able to make the decision to exclude such people, and if this decision has been made regarding your son there isn't anything you can do about it except try to pursuade them to make an exception, which it appears they are unwilling to do in your son's case.
All I can suggest is that as time passes and the length of time since your son's offence lengthens, they may be more inclined to grant a visa despite the offence, but there are no guarantees.
I appreciate this isn't the answer you were hoping for, but it does reflect the legal reality of your situation.
Please rate my answer.