shish, I dunno why he's doing the night feeding thing....babies just do odd things from time to time, with no consideration for their poor mothers
I sometimes suspect, however, that this is behavioural, rather than purely nutritional/hunger (though in a five month old all this may be one thing).
You see this sort of behaviour (change in feeding habits, either frequent feeding, nursing strike, fussy feeding) at times when the baby is under some expectation to change or picks up some pressure. You see it in babies whose mothers have gone back to work, mothers who are trying to wean, mothers who are struggling with the baby to take a bottle, mothers who are trying to get the baby to feed to their (mother's) agenda....I think it's not a crazy idea to suggest these babies are sensitive, and aware, and because feeding/sleeping/comfort is so very central to their lives, they express their response to this by changing their feeding/sleeping/comfort behaviour.
If toddlers or children do this, we think immediately of what might be going on in their lives to cause it. I don't see that it might be so very different with babies.
If you stop the struggle and accept that when the time comes, yes, of course mumsnet (and moi!) will be here to help you wean gently and comfortably, and you go with your baby's flow, you might well find he relaxes and trusts that he can feed as and when he wants, and he won't express his needs at night so much.
Please believe me when I say I am speculating here, and that I don't think any of this is conscious in your baby, or that he is in anyway 'unhappy'. But there is some research from infant mental health specialists to show that babies' behaviours are indeed an expression of their psyche, and that he may well be actually finding excellent ways to cope by increasing his night time demands; he has a lovely mum who responds to this, as well