To help our DS' through winter we give them a daily dose of inner health plus for kids in their yoghurt & the kids vitamin pentevite. They say a healthy immune system starts in the gut & so far DS1 has only had one lot of runny nose & cough. DS2 on the other hand gets it back every month. We are just starting again but he's just turned one and building his immune system. I am not sure if you can get something like that where you are (probiotic) it may help.
Does he prefer cold food to hot food? What sort of foods does he like to eat? Can you get a vitamin supplement like Pentevite? Sorry I am not much help the above suggestions are what I would think of for my boys as a starting point. If he is low in vitamins & minerals he will get tired more quickly & take longer to get better. I am sorry you can't get the answers you need where you are. do you have any plans for holiday back here?