OH Lorraine you poor thing! You have really been put through the ringer lately.
I agree with all the advice given but Debbie is right about your other cats being healthy and chances of them getting sick are slim and even if they do they probably won't get nearly as down as this little guy. We have had tummy viruses run through the whole house and it's just a matter of waiting it out. You are doing everything possible to prevent it spreading but I really don't think you need to worry. Everyone, including Popeye, will be fine. I have seen kittens look like they are knocking on death's door and they come back around.
Keep doing what your doing and we will keep sending PT prayers. I have no doubt that Popeye will be fine. He has had a stressful life up to this point and it's going to take a bit before he recovers from everything.
We love you Lorraine and you are taking excellent care of the baby. Magoo sends {{{{{HUGS}}}}} to you and little Popeye. He said to tell you that he had the nasty URI one time too and he's ok now.
Hang in there Lorraine and one day when Popeye weighs 15lbs and is lounging around in the window this will all be a bad memory.
Lots of {{{HUGS}}} from me to you.