Lack of correct preparation is at the root of most such problems because the affects are often only apparent a long time after the work has been completed. The explanation your decorator has put forward is beyond ridiculous and merely demonstrates that he doesn’t know what he’s talking about.
In the first instance I would take photographs of the worst affected areas and write to your decorator (with copies) explaining you are not satisfied the work has been carried out properly. Set a time limit for him to reply and to either offer to put the work right or to compensate you so you can arrange for someone else to do it.