I think it would be appropriate to say something especially since it was in the kids bathroom.
Remain calm at all cost when bringing it up cause I am sure that it could only do more damage if you get angry and try to discuss it.
Also I don't know how hard it would be to wait and bring it up when he isn't high but it seems it would be best to approach this with him when you know he is straight.
I hope you can set a boundary that you know you can adhere to. ((((BIGHUGS))))