The dress codes have been relaxed for all the signature restaurants. It now basically says no swimsuits, you can pretty much wear anything. My DH usually wears dockers or nice shorts & a polo or Disney Hawaiian shirt to resort signatures. We’ve seen jeans, park attire, nicer casual & very rarely someone in a sport coat or suit. We ate at Monsieur Paul in January. My DH had black dockers & a button down shirt, I had black slacks & a dressy top. Only 1 other couple was dressed similarly. Most people were in shorts & park attire, a few other couples with the men in shorts & polos & the women in capris & nice tops. Your DH will be fine in dockers & a polo.
The exception to this is Victoria & Alberts, I’ve never been but I believe they’re the only restaurant that is strict with their dress code.