Please do not attempt to trim his beak yourself. There is no way to know where the quick is and he can bleed to death if you cut it. And without the proper tools, the beak may splinter and cause worse problems. You need to take him to an avian vet, who will have the right tools do do this correctly. This would be caused by either a deformity in how he closes his beak or a nutritional deficiency. I would suggest discussing nutrition with the Vet to try to keep this from happening again. Or if the Vet determines this is due to a defect, then regular beak trims would be better than allowing the beak to become overgrown.
Thank you for asking Lafeber,