Jock, I suspect our American version of "semi-formal" is essentially the same as what you think of as "smart". There are others here, I'm sure, who can better compared the two. However to me, I believe them to be essentially the same.

To the OP. I would return to the groom. Clearly he would desire his new father-in-law to be dressed properly. I suspect if you ask again with some guiding questions, you will get your answer.