Architectural blueprints have been explicitly covered by copyright in the US since 1990, but were likely implicitly covered before then.

He could always provide an open-source license before claiming that he is "open sourcing" his designs. You could always check for a license before claiming that something is open-source. Putting the onus on people after the fact to make those previous claims true doesn't make any sense.

Yes. Under 17 U.S. Code § 102 (a) (8). Blueprints might be under (a) (5) too but I'm not an expert.

(a) Copyright protection subsists, in accordance with this title, in original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device. Works of authorship include the following categories:

• (1) literary works;
• (2) musical works, including any accompanying words;
• (3) dramatic works, including any accompanying music;
• (4) pantomimes and choreographic works;
• (5) pictorial, graphic, and sculptural works;
• (6) motion pictures and other audiovisual works;
• (7) sound recordings; and
• (8) architectural works.

https://www.law.cornell.edu/uscode/text/17/102

We should aks him to publicly state the license recommending CC BY-SA, yes

Link?

It absolutely can be, and there are plenty of do it yourself plans out there. They just tend to be pretty expensive. It's nice that there's something out there now just completely open.