It means he looks very innocent,as if he could do nothing wrong.

If someone looks as though 'butter wouldn't melt in his/her mouth' means that a person seems to be very pure and innocent and incapable of doing something bad. Originally it did mean that a person was aloof and cold, however the more modern meaning is the first one. Do you know what ' a wolf in sheep's clothing'?....this is a person who looks nice, but they are very dangerous. A wolf in sheep's clothing might look as though butter would not melt in his mouth! :)