Was it bedtime when he screamed last night? 9 months is commonly quoted as the age when they start to experience separtion anxiety - maybe he is now asserting an opinion on being put to bed? If he seems angry rather than miserable and whingy?
Temperature? is it worse when he lies down? (a common factor in earache)
Anything! As suggested teething, cold etc. Could be brewing something, a random virus, an ear infection, generally under the weather. I'm with the calpol brigade on things like this, if it is totally unlike him and he just isn't himself then there is probably something that hurts so calpol and a cuddle.
That was the age we did controlled crying with DD1. Probably I will get shouted at for suggesting that, but it was the fashion at the time! Works too - provided there is nothing else wrong with them. Took 3 days with DD1, never had to do it with DD2 because she was always put to bed awake, door shut, and don't switch monitor on for at least 5 mins!