Is he covered with it..?
A 1 minute FW
or even 30 seconds should make a lot of the visible spots drop off...,but it may be too much for it to handle...I also think leaving it be will be a certain death ...
The water has to be same temp ,,,that is all i ever worried about...some say same PH too,,but for a quick dip i don't think it matters..when and if you do hypo it will..
Your going to have to use your own judgement ,,but a FW
could give him some instant relief IMO