God I'd cry if my mum just buggered off and left me in my cot in the dark without any means of comforting too - at his age I mean!
Look, you don't know what's going on, he could be teething, he could be lonely, but while leaving him to cry it out will teach him 'no-one will come, there is no point in trying to get attention' you are also teaching him some other stuff you may not be so happy about.
May I suggest you invest in a copy of the No Cry Sleep Guide and read it and see the many hundreds of ways you can solve this, without either of you having to get upset?
You don't say whether this has always been the case or not, if it isn't, there is probably a (solvable) reason for it. You just need to figure out what it is - perhaps he's too hot, too cold, in pain, or has indigestion (if he's only happy sitting up on your lap)?
Cheer up, you can fix this and you are right to want to do it without upsetting either of you.