If he looks like he prefers one boob to another, and if you tend to use the same hold (the one with the baby lying in front of your body - switching him round to swap boobs), then try fooling him by using the "rugby ball" hold (i.e. under your arm, so that the same side of his body makes contact with yours despite the fact that he is on the less preferred boob, IYSWIM).
If he is ill, I don't see how switching to formula will help, as breastmilk is more easily digestible so doesn't need to be in their tummies for so long before it starts being digested (that's one of the reasons that BF babies need to be fed more frequently than FF ones). As such, if they are ill, and bringing milk back up, breastmilk is more likely to have a chance to be digested, partly. Also the antibodies in your breastmilk will help him combat any illness more effectively.
Yes, it is really hard work BF-ing, and you would not be the first to pack it in at this stage if you decide to do that. But in the long term, if you can get some advice from someone more local (and more expert), then you only have to stick this out for another few days before your DS will turn a corner and you will find it getting easier again.
Good luck, and well done for persevering with one of the most challenging aspects of new mother hood (and that's a well-kept secret!)
PS com is good for advice too.