Is it appropriate to express your confession by SMS?

Not suitable, because SMS has many shortcomings.

First, the tone may be unclear, which will cause misunderstanding.

Second, it must be vague.

Third, and most importantly, treat the person you like directly, so as to get the effect!

You should let him know that you like him. You don't have to be shy to like it, and it doesn't mean it's not good to say it!