My two cents is, I would try to take the horse back. I get the terrible circumstances right now, but the way I see it, if you leave this horse with his current person, he WILL founder and he WILL suffer, possibly have to be PTS. Whereas if you take him back, you MIGHT be forced to evacuate but be unable to do so, and he MIGHT suffer and die.
In other words, looking at it objectively, there is a greater chance of him coming to harm if you leave him where he is. Even if you take him and then can't evacuate him (even if you just have to turn him loose and hope for the best), that may be better than what's about to happen to him where he is.
Regardless, my heart goes out to you, being in this situation. Neither choice is easy.