Hi Jasmine 👋,
To avoid sounding flirtatious, you could just say:
'You're a good guy'
'You're a really nice person'
'You're the best!'
Alternatively, pick something about the person that you like. E.g.,
'You're really kind / generous / thoughtful / funny.'
Just don't say 'handsome', or he'll definitely think you're flirting, ha ha 😊