I've found
this site good for the lycine powder which you can mix in with their food. You can buy echinacea liquid from most chemists,
Holland and Barrett also do it.
I believe it's possible that too many antibiotics will weaken the immune system so these may be worth a try.
There's also something called
Moducare which I'm currently trying and
Transfer Factor (which I haven't tried yet).
I've found that the first two seem to help but can't speak for the last two yet.