@Karenah86
Thanks everyone for your replies. There's definitely no other woman but I get why that would be people's first suspicions. We talked some more tonight but I'm still quite confused and at a loss as to where we go next. I asked if he wants to make things work and he said 'I think so.' Not quite the affirmative answer I was after! He thinks he's the problem - he's unhappy and therefore isn't making me happy. But he doesn't know what we can do to help him be happy again. He said he is still in love with me and that he'll always be in my life because of our son. It seems in one breath he's talking like we should separate, but in the next it's like he wants to make things work. Can you see why I'm confused!
This really isn't acceptable behaviour op. Of course, everyone can become confused about a relationship but any decent person keeps that to themselves until they've figured out what's wrong. Once you have done that, you can then discuss genuine issues with your other half and try and work it out, or it's too hard and you can't but at least you've tried.
What you can't do is jump about from one position to another, confusing the other person and not being clear about whether you want to stay in the relationship or not. He is being confusing because he's already decided he wants out but can't say that straightforwardly because he is afraid of hurting you, or he doesn't want to look like the bad guy or there is another woman on the scene. Sorry if that sounds harsh because I know this must be hellish for you 
I wouldn't be allowing him to second guess me with that "he's unhappy therefore isn't making me happy" clap trap either. Sorry but that is very manipulative because:
- You a grown woman who knows their own mind and you don't need a man to tell you whether you are unhappy or not.
- If he's that unhappy then he should just say it and leave for his own sake rather than making out that he is "too honourable" to stay because he's making you unhappy. That's bull, sorry!
In your shoes I would be taking the initiative here and taking back some of the power. Why is it only him who gets to decide what happens in your relationship? If a man said "I think so" to me I would be packing a bag for him and asking him to leave until he was clear in his own mind.
You are worth more than this op! You need to grey rock this one. Find some anger. Just tell him you've thought about things overnight and he's right, and you'd like some space.
It will be hard but don't allow any more discussion, certainly do not plead, don't show any upset in the morning, just insist that he goes away until he either comes running back or he doesn't. But he doesn't get to stay and enjoy home comforts and experience all the benefits of marriage while making you feel crap.
None of us are perfect, but ime too many women in these situations automatically jump to the default position that somehow they must be to blame. This is usually utter bs! Men imho very rarely indulge in such introspection or genuine self examination about how they could be doing better. I know that's a massive generalisation, but I've seen it so many times.
Keep strong op