9yr old constantly ill over winter - any suggestions?

[Miji]

My son goes through every winter never really getting over anything. Even head colds last for weeks. He eats well, sleeps and is very happy but looks pale with dark rings under eyes. Constantly says he doesn't feel well. Blood tests normal and already seeing cranial osteopath to boost immune system. Anyone with similar problem? I know by summer he'll be fine but he is missing too much school over winter.

[Trophywife]

my sister used to be like this as a kid, in the end the only thing my mum could find that made a diffrents was to give us all an extre dose of cod-liver-oil and vit C as soon as it started to get cold.
HTH