If he is just looking to re-sit and not to retake the year then there's no need for him to stick to the same exam board as the entering centre normally uses. All the English exam boards have to comply with Ofqual requirements for content so there aren't big differences but WJEC is not bound by this.
WJEC still has modular A levels but the English exam boards are linear so there would be some issues swapping between them, particularly as I don't think you could transfer the pass at assessed practicals from linear A level to WJEC, I don't even know what WJEC does about assessed practicals. However, you can transfer the pass at assessed practicals between AQA, OCR and Pearson Edexcel if you swap boards between them.