I think he may be refusing more because he is unwell, and not necessarily because of the antibiotics. I have given Paige the fast-Tract like Kelly suggested when she recently had antibiotics to help her little tummy deal with it.

Try not to worry too much about quantities he is having, but mainly try to keep him hydrated and offer small feeds regularly. You will find that once he is on the mend, that his appetite will be back with a vengeance and you will be feeding him more often.