After looking at the wiki page for borr to look into translating the commentary for this image, I have some issues with how several of the images on danbooru of this character are tagged.
To start off with, the character depicted is canonically male. Source: http://ssssgridman.wikia.com/wiki/Borr Therefore, in my opinion, this post should be tagged with 1boy and otoko_no_ko while the 1girl tag is removed. It could be argued that if an artist is misinformed about the sex of the character being depicted that it is then effectively, albeit unintentionally, an illustration of a genderswapped version of the character. However, I do not believe that applies to this specific post. In the artist's commentary, Borr is identified as seeming like an "otoko no ko" (meaning male child, different from the otoko_no_ko tag), but not seeming like a shota. Additionally, the title of the commentary, "Borr-chan-kun-san", seems to imply that the piece is intended to look androgynous, rather than genderswapped.
On a slightly different topic, I'm not sure whether the loli tag should be removed. Having never dealt with traps before, I'm not sure what the convention is. Very strictly speaking, the tag definition would imply that the loli tag does not fit. It reads, "This tag is for sexually explicit or sexually suggestive art work of girls who appear to be young or preadolescent". However, given the emphasis on how the character appears over the canonical truth and danbooru's policy of "tag what you see", the correct usage of tags here eludes me.
I haven't edited the tags at all for now in the hope that someone with more tagging knowledge will come along and resolve this issue correctly.
After looking at the wiki page for borr to look into translating the commentary for this image, I have some issues with how several of the images on danbooru of this character are tagged.
To start off with, the character depicted is canonically male. Source: http://ssssgridman.wikia.com/wiki/Borr Therefore, in my opinion, this post should be tagged with 1boy and otoko_no_ko while the 1girl tag is removed. It could be argued that if an artist is misinformed about the sex of the character being depicted that it is then effectively, albeit unintentionally, an illustration of a genderswapped version of the character. However, I do not believe that applies to this specific post. In the artist's commentary, Borr is identified as seeming like an "otoko no ko" (meaning male child, different from the otoko_no_ko tag), but not seeming like a shota. Additionally, the title of the commentary, "Borr-chan-kun-san", seems to imply that the piece is intended to look androgynous, rather than genderswapped.
On a slightly different topic, I'm not sure whether the loli tag should be removed. Having never dealt with traps before, I'm not sure what the convention is. Very strictly speaking, the tag definition would imply that the loli tag does not fit. It reads, "This tag is for sexually explicit or sexually suggestive art work of girls who appear to be young or preadolescent". However, given the emphasis on how the character appears over the canonical truth and danbooru's policy of "tag what you see", the correct usage of tags here eludes me.
I haven't edited the tags at all for now in the hope that someone with more tagging knowledge will come along and resolve this issue correctly.
The rule of thumb is that what is visible in the image is what's worked with. For otoko no ko, there is a requirement that they are visibly of the male sex or, if too ambiguous, canonically male. Creator notes and interviews are generally disregarded for this and the tomboy tag, such as when the Attack on Titan creator pretended for a week that Armin was a girl all along. In short, because Borr-kun has yet to be properly identified, there will be an influx of images reflecting artist perceptions, but once it is revealed, the ambiguous ones should be retroactively tagged to match.
For otoko no ko, there is a requirement that they are visibly of the male sex or, if too ambiguous, canonically male. Creator notes and interviews are generally disregarded for this and the tomboy tag, such as when the Attack on Titan creator pretended for a week that Armin was a girl all along. In short, because Borr-kun has yet to be properly identified, there will be an influx of images reflecting artist perceptions, but once it is revealed, the ambiguous ones should be retroactively tagged to match.
I suppose it's fair enough to say that his/her gender hasn't been definitively identified yet. However, you bring up that tags before that point should reflect the artists' perceptions. As stated before, the artist perceived themself to have been drawing a male. The perception that this depiction is of a female belongs to either the uploader or the tagger. As such, shouldn't they be changed until Borr's gender is definitively identified?