What I would do is pop it the medical way so it doesn't leave scars so it at least doesn't look that visible. But you don't have to do that and no don't cancel that date, just use some makeup to hide it and it's no biggie even if he saw it!
Nah, don't cancel - unless you're willing to tell him the reason why! Otherwise he might start thinking you aren't actually interested. You may as well go right? Be confident and try not to worry about it :)