Hello sir. It seems to be writer's cramp, however offline examination or a well guided video is necessary for proper diagnosis.
1. Using a pen with free flow like gel pen and with wide grip ( thick pens ) will be beneficial.
2. You can shift to typing , if possible. Using the other hand for writing is also suggested, but it is difficult for most people.
Regards
Next Steps
Please consult a neurologist for further management. There are some medicines and also botox inj available.