The concentration (%) affects the dose, not the amount in the bottle (which I think you mean by 5ml or 10ml solution? Sorry if I'm getting confused!), so the dose should still be 0.1ml. If he has not had any other pain meds you can inrease it to 0.2ml (Vedra's advice on Metacam as pain relief), or just give 0.1ml twice a day.
The dose can even go up to 0.3ml but I wouldn't give that dose to a pig as lightweight as Boris unless I knew it would be purely a case of keeping him comfortable for his last days.
I'm pleased to hear he has picked up a little, to know that he is munching on some food is a little relief. It makes it a thousand times harder to make a decision when they keep picking up now and then. The active ingredient in the Zithromax is azithromycin - if that helps any.