Hello, everyone!

Please read these sentences below.
a. He looks like a Japanese.
b. He looks like Japanese.
Which is correct, a. or b.?
'He' is a singular pronoun, so it would be safer to say 'a Japanese.'
However, I think...'I'm Japanese' (instead of saying 'I'm a Japanese) is correct.
If so, b. is also acceptable, I guess.
Please help me.
Thank you in advance!