As hearbreaking as it is, the only way you are really going to get round this is by letting Jack cry at bedtime. Keep to his routine but let him cry for 15 minutes. Then go in, dont say anything, just lay him back down making sure he has any blanket or teddy he may use as a comforter, then walk back out. Dont go back in again for another 15 minutes. Keep doing it til he finally settles down and goes to sleep. I know its very hard, I went through the same with my daughter at about the same age. I used to have to put on headphones and listen to music, leaving her father to listen out to make sure she was ok, as I couldnt bear to hear her crying either.
Bedtimes were very tough and tiring for us all for about a weeks or so, but she soon learnt that she was going to go to bed with or without the fuss. On the nights she was going to sleep with no or a little fuss, I would make sure I gave her lots of praises and cuddles as soon as she woke the next morning letting her know it was for being such a good girl and going to sleep.
Its tough, very tough, but it does work if you persevere with it.
Good luck :)