I would first make sure there isn't any medical reason going on, take him to the vet and make sure there isn't a UTI. Also, it is not uncommon to have relapses. Some may take up to two years before you can totally 100% trust them. Some are trained early and never go back.
Also, there is the possibility that he is starting to mark (yes this DOES happen in neutered males :
) I would start from the beginning again, and watch him like a hawk, whenever he tries, take him to the litter box. There are also belly bands that are helpful if you can't watch him like a hawk