My 18 month son has always been an appalling sleeper (has never slept longer than 2 hours in his life) but in the last two weeks he has become even worse! Can anyone help me fix him? 
Until two weeks ago we had a reasonable routine where my DH would do his bedtime routine (starting around 7pm and taking around 30-45 minutes) and I'd then feed my DS to sleep and transfer him to his cot where he'd sleep for about two hours. This would be enough time for us to have a bit of an evening to ourselves and watch TV/read a book etc.
When my DS then woke up around 10pm I would then bring him into my bed and we cosleep until morning. He typically only wakes 2-3 times a night when he's in with me.
However... For the last two weeks my DS has fed to sleep as usual but we just cannot transfer him to the cot. As soon as he touches it he's screaming. I've tried holding him for ages (up to 20 minutes) to make sure he's properly asleep but it makes no difference.
After I settle him back to sleep we repeat the whole thing several times before my DS becomes overtired and starts giggling and asking for a story/toys/generally pratting around and not being sleepy at all. At that point we generally give up as it's getting too late to bother doing anything before bed, or we bring him downstairs, or I'll just go to bed and bring him in with me.
As a result we've had no childfree time for a fortnight now, and I have literally no time to do anything in the evening.
He did have a really bad cold a few weeks ago, could this be making him super sensitive to being left alone? Or is it some sort of developmental leap? (He's having a vocabulary explosion at the minute, learning a new word a day, could it be that?) Should I just wait it out?
Any help gratefully received.