This does happen, and I wouldn't be too worried unless it becomes a habit. It could be that he got spooked (something moved or he got nervous) and simply regurgitated. Sometimes, if they are moved, or their cage is tampered with, right after eating, they may regurgitate. It's not likely to be something serious if it is the first time. I would wait about 10 days before reoffering food to him. Make sure when you do feed again to not disturb him at all if possible, and it might help to put a cloth or towel over his cage while he's eating so that nothing can spook or disturb him and make him regurgitate. I hope that helps!