Can the posters who think OP should complain and be butthurt about her son not being allowed to join explain how exactly they think that will benefit feminism?
And would you really fight for your daughter to get into a one day engineering course where there are only males who you know do not want her there?
Would you really want your teenage daughter to spend a day with men who, including the course leader, are offended by her presence? And if not, where is the difference to that sewing course?
The key difference is: they are either unpaid, or members only. They exist as spaces for women as much as to teach a skill.
So a woman can only give sewing courses for women who would rather have it be women only, for whatever reason (such as trying on clothes) if she can afford to donate her time for free?
Men, as a class, are not banned from learning how to sew. They can access this skill easily and freely almost everywhere. It is nowhere the same as men preventing women from going to university to learn male dominated professions. Hell, clothes-making used to be a male only profession, with all the boring and tedious work outsourced to women and the design being done by men.
I truly cannot see how it would benefit feminism to allow a teenage boy into a sewing course, be it a one day one, a for profit one, or a weekly non-profit group.
So why do you post this on the feminism board, unless you want to talk about how it is problematic to push males into spaces that used to be women only for whatever reason?