"Whenever my 9-year-old sees anyone else get praise, he feels bad about himself. How can I help him improve his attitude?"
Sounds like a good time to have a discussion about the Golden Rule.
Your son must learn that, to make and keep friends, he has to be a friend. That means looking for the good in others and giving compliments.
Competition drives us to do our best, as long as it's tempered with compassion. Does your son play a sport? Ask his coach to reinforce your efforts on the home front with a pep talk about what it means to be a team player.