What perplexes me is how he can be such a mother's boy, demanding her attention every second yet he's so unruly towards her ….... except when the situation has to do with the 3 of us. Then he's all on mum's side (as though there's friction between us, which there isn't), openly favouring her against me. I understand nothing.

It would seem to me that you have to try to be more of a friend to your son than a father.
The many rules we weigh our offspring down with are mostly not really necessary and only serve to put distance between us.
Do it now before it is too late to recover the situation and you grow too far apart to close the increasing and irrevocably developing gap.

We have what I assume is the classic father-mother relationship with our (now) 8-year-old son. My wife spoils him (although not in the extreme - yet) and allows him to test the limits of her patience. I set the rules (fair ones, if you ask me) and stand by them. I usually have enough tolerance to pretend I don't see it when he pushes mum well beyond what most parents will stand for, although I do occasionally interfere on her behalf. Anyway, when it's just he and I things go smoothly - very well in fact - but when she's at home the two of them argue over just about everything. My wife has never learned how to avoid perpetual disagreement but rather replies in such a way that offers room for counter-point. They can go at it for half an hour! Over nothing at all!

But what is really worrying for me is the jealousy our boy displays when my wife is at home. He insist on holding mum's hand (demonstratively declaring he won't hold mine) when crossing the road, he keeps her between us whenever we're in town, and a hundred other, small displays of “mother's-boy”-ish-ness. If I were a stranger I'd think I were an awful father who instills fear (or worse) in the boy. Then there are the cuddles my wife and I share … he comes rushing in to give mum a cuddle, to outdo me I suppose, and if he hears the two of us in discussion he'll come charging in the room to speak over our voices and interrupt each and every sentence in turn.

So OK, I realize the problem lies with my wife's inability to draw a few lines in fostering the boy, but she just isn't made that way, the dear. We have talked about it and she usually says that she quite simply doesn't want to alienate our son. Yes, that's right – she's afraid of making him angry with her. She's worried he won't like her.

I'm torn. Pressuring my wife to change would be putting too much stress on her otherwise lovely philosophy on life, yet trying to get our boy straight without it might be impossible.

Has anyone else had this experience? If so, what long-terms effects has it had on the child?

This is what to do .
Go out and buy Steve Biddulps book
Raising Boys
It will tell you exactly how to handle this .
At the same time buy his book Manhood
Every man should read it and especially every father of sons.
( you can pass it on to him when hes bigger)