What should i do to work with same stamina for whole day?

It is possible only if he is healthy and mentally strong. His brain shouldn't get fatigued. For that you have to sip water every now and then if you cannot drink in intervals. If the body gets dehydrated, the brain fatigue resuktsand you won't be able to work properly. You have to have proper breakfast in the morning and a light lunch. If you have a heavy lunch also you'll get mentally tired. see that your Vitamin C level doesn't come down. Look after your health and you can do that. The main thing will be that you should love your job and have full interest in it. All the best.