(Question)
My son was in the back yard and emerged with hives. Is this poison ivy?
(Answer)
Thank you for contacting the Toronto Master Gardeners with your question.
It is a bit hard from the picture, but it appears to be Goutweed, which is very invasive.
Please find the following references on how to deal with it;
https://www.gardeningchannel.com/how-to-get-rid-of-invasive-bishops-weed-goutweed/
Good luck with it.