Well I think it's sweet that he feels confident enough to tell you that he thinks you'd look lovely in a dress, it's quite different to saying YOU MUST do so. And he hasn't slagged off your dress sense at all, has he?
The lingerie dept in M&S is full of stuff designed to look sexy, by which I mean attractive NOT submissive. I'm a dresses fan. Confidence can come from looking good, that's why make up and clothes shops sell us an awful lot more than any of us actually need to survive.
My first thought was, are there any celebrities/people in the public eye who you share a body shape with? You could check out the styles they wear (google them using Images) and see if any 'look' takes your fancy.
I'd avoid anything waisted if your tummy is bigger than your boobs, but a slimming sheath dress (aka tunic!!) with a cardie, tights and flats is an easy look. Monsoon does nice dresses and a couple of hours in any department store trying things on will give you a much better idea of what suits.
You sound like you might be a bit fed up of wearing the same thing all the time and say you 'never look great' so maybe he thinks you would FEEL great in a dress. And having your DH look at you like you are a foxy mama would not be a bad thing.
Amillionyears, have you got a spare fire blanket?