Actually it doesn't only state dress clothes as the only appropriate attire, it states no jeans, ( leaving it open to interpretation of what is then "accepted", no flip flops and no tennis shoes, so obviously some have thought those where appropriate before.
Regardless, we are going to dinner here and I guess we will see if anything is said to him, it shouldn't be from what I've heard others say they have worn: ie) khakis, dockers, etc...
It's what he's wearing and I'll be sure to post if there are issues so others spending time worrying won't have to in the future. Then again if he gets turned away I guess we will have to book another night and "rent" dress pants