Trouble with not being accepted by Evri is that your son could get a late dispatch defect.
Did buyer buy the postage label, if so whether they can cancel it and pay your son RM postage, There is no way through ebay for paying anything extra, and buyer is not boing to think it is his problem.
In future can your son not avoid using evri, on the “simple” postage and only offer Royal Mail, not giving buyers any choice.
Whoever dreamt up this complicated system, obviously does not know what simple means.