Gosh, I think making you make a decision by Weds is a bit harsh. If he has a brain injury, it would likely take longer than 7 days to improve (if it is going to).
I would still err on the side of caution on this one. If Mr Miaow is happy in himself (eating/ toileting/ purring) then I wouldn't- he may need a little extra attention for a bit, but as long as he's not in pain why not let him carry on?
It's a really hard position to be in and a difficult decision to make. I had our old family cat PTS a few years ago and I'm still gutted when I think about it. Different situation though, he was old and really quite unwell- many, many things wrong with him.
I'm so sorry your family is going through this 