Winning a prize is a very satisfying experience. On such happy occasions one wants one's happiness to be shared. Thus, by sharing his/her happiness you can inspire the individual to win more laurels.
First of all, express your pleasure:
1. It's very exciting/delightful to learn that you've come in the merit list of the Higher Secondary Examination.
2. The exciting news that you've won the first prize in the competition organised by the Indian Literary Academy, has filled us with joy.
3. It's very thrilling to hear that your university has named you the 'Sportsman of the Year' and awarded you a gold medal.
4. How delighted I am to know that you've been awarded the scholarship for higher studies abroad I
5. It's difficult to express in words my joy at your getting first prize in the All India University Elocution Competition.
6. I'm as happy as you, if not more, at your coming first in the International Children's Painting Competition.
7. It was really great amazing to know that …………………
8. We were proud to know/ hear that you won the first prize …………………
9. Felicitations on your having won the first prize in the ………………… competition.
10. We were sure that you could make it happen.
Now give your heart-felt congratulations:
11. Please accept my heartiest congratulations on this glorious achievement.
12. Please convey to your son my hearty congratulations and blessings on his great achievement.
13. Please accept my hearty congratulations on securing a higher position in the merit list.
14. My wife/ mother join me in congratulating you on this happy occasion.
Praise his merit for winning the prize:
15. In fact, your splendid performance and the subsequent reward haven't come as surprise, because I know that you are an ace debater/painter/athlete.
16. I was sure that a brilliant boy like your son could definitely top the list.
17. This recognition of your talents was, in fact, long overdue.
18. This is another well-deserved addition to the prizes you have already won
19. Your hard work and passion for perfection alone could bring such reward in hand.
20. It was heartening to see you rise unbeatably in the competitions.
21. You have made your parents and friends proud of your achievements.
Discuss the expected good results of winning the prize:
22. With a gold medal in hand it won't be difficult for you to manage a scholarship for higher studies abroad.
23. This award has, of course, confirmed your reputation as a writer.
24. These are the landmarks that will get counted in your long span of career.
25. Having won this high recognition, you can now make a name in the national as well as international competitions.
26. It's really fortunate of you/ You are really lucky to have won this scholarship as you will, now, be able to continue your studies without any financial difficulty.
27. With the help of the prize money/award money, you will be able to go ahead with your creative writing without any financial problems.
28. With this excellence in debating, you might well turn out to be an important public speaker in future.
Make this occasion a source of happiness for everyone in the family:
29. Your parents must be feeling very proud of you. You have made your parents really proud.
30. All your friends here are extremely proud of your distinguished achievement.
31. You must be feeling really proud of your son.
32. How we wish we were as talented as you!
33. My family is overjoyed at your impressive performance.
Once again, extend your good wishes at the end of the letter:
34. This is just the beginning and many such awards will follow you.
35. I wish you to add many more feathers to your cap.
36. I hope your very talented daughter achieves many such distinctions in life.
37. (I) hope you continuously strive to better your record.
38. Best Wishes for a distinguished and bright future.
39. Keep it up and you will reach the peak of fame.
It is very exciting to hear that your university has named you the Sportsman of the Year' and awarded you a gold medal. Please accept my heartiest congratulations on this glorious achievement. In fact your splendid performance and the subsequent reward have not come as a surprise because I know that you are an ace athlete. Having won this high recognition you can, now, make a name in the national as well as international competitions. Hope this is just the beginning and many such awards will follow you.
My best wishes for a distinguished and bright future.
When you write too personalized a letter, you could use a usage like one of the following according to the context:
Congratulations....., Fabulous...! Great..... !
Wonderful...! Amazing..... ! Fantastic.......!
Terrific.....! Marvelous..... ! Miraculous.........!