You will believe a man can fly, but hiding his suit under his regular clothes?
Impossible to believe.
I know it's a ridiculous observation, but suspension of disbelief relies on getting the real world stuff right so we swallow the outlandish stuff. Otherwise the house of cards collapses. It's like making sure his cape flaps realistically, we know what that is supposed to look like, so if that looks fake we don't believe he can fly.