Quote Originally Posted by GatorUK View Post
Strictly speaking semi informal is Black Tie with gentlemen's attire as follows: Black dinner jacket or white in summer (tuxedo), black trousers with one satin seam on the outside leg, black vest or cummerbund, black bow tie, white silk scarf, black or gold cufflinks and studs. Formal would be a white tie event. If suits are required that is what is know as evening informal. Gentlemen's attire is normally a business suit, necktie, lace-up shoes, and for evening occasions a non-button-down collar dress shirt. A sports coat would be considered casual.
This may have been true, once upon a time. There's no way a sports coat is generally considered casual. If he shows up dressed as what you call "semi-informal," he'll be way overdressed.

I agree with those further up. Get a jacket.