Wow, that's amazing

[Silverobama]

My friend Kuma told me he changed jobs. His previous job was a good one, and many people advised him against leaving because no one could be sure his current job would meet all his needs like the previous one did. However, the current job is Kuma's dream job, which he has longed for. When he told me he finally found this job, I said, 'Wow, that's amazing.' But on second thought, I realized that wasn't quite natural because Kuma wanted validation for his choice. He wanted others to recognize that he had made the right decision. What can I say instead in this context?"

 

[emsr2d2]

My friend Kuma told me he has changed jobs. His previous job was a good one, and many people advised him against leaving because no one could be sure his current new job would meet all his needs. like the previous one did. However, the current job is Kuma's it's his dream job, which he has longed been longing for [for years]. When he told me he had finally found this job, I said, 'Wow, that's amazing.' But However, on second thought, I later realized that wasn't quite natural because Kuma wanted validation for his choice. He wanted others to recognize that he had made the right decision. What can I say could I have said instead in this context?"

Please note my corrections above.
How do you know he was looking for validation? What did he actually say to you? As it stands, I can't see anything wrong with your response. It shows you're happy for him and that inherently suggests that you approve of his choice.

 

[Silverobama]

Please note my corrections above.
How do you know he was looking for validation? What did he actually say to you? As it stands, I can't see anything wrong with your response. It shows you're happy for him and that inherently suggests that you approve of his choice.

He later told me that his parents and some of his friends didn't want him to do his "dream job" but stick with the "previous stable job" but he didn't want to. When he told me that, his voice was full of doubts (whether he could do his dream job well/whether his dream job was worth his sacrifice)

Is it natural to say "That sounds like a great opportunity!" or "I'm really happy for you!"?

 

[emsr2d2]

He later told me that his parents and some of his friends didn't want him to do his "dream job" but stick with the "previous stable job" but he didn't want to. When he told me that, his voice was full of doubts (whether he could do his dream job well/whether his dream job was worth his sacrifice)

Is it natural to say "That sounds like a great opportunity!" or "I'm really happy for you!"?

It's too late to say "That sounds like a great opportunity!" That would be something you could have said when he told you he'd spotted a job ad for his dream job and given you some details.
You can certainly say "I'm really happy for you!", although I can't say that that 100% gives validation for his choice. You could say "You definitely made the right choice/move!"

 

[probus]

Both of those are perfectly natural.