Hun, what a great idea! I know of other mums who have done a similar thing. EBM at any stage contains a great range of nutrients, and so many biological factors we don't even know what they all are. These are beneficial at any age - there are even occasions where EBM has been given to sick teenagers and adults (eg with cancer) and there has been a big improvement. It is impossible to know how much your DS's immunity will be improved, but it will provide some extra protection and will definitely not do any harm.

Often expressing works best when you do it at a time that works best for you. But in order to disrupt your DD's feeding as little as possible, perhaps express from the second breast just after she has fed. You might not get enough EBM for a bottle in one go, but as long as you cool both batches first, you can mix it together until you have enough. You could also give a small bottle of EBM and then give him milk to finish his feed if he hasn't had enough (or just reduce the amount of milk he drinks for now). It is best not to actually mix EBM and milk (or formula) in the same bottle or cup though.