Hi Maria
The only thing that worked for me was this.
Lay him down,go out of the room,count to 10,go back in,lay him down,go out of the room,count to 10,go in etc
Only go in if he's crying.
The first couple of nights are the worst.Took 50 minutes for me the first night,but works a treat now.Although I very rarely have to do it now.
The crying is very bad to begin with,but persevere.
You need to keep this up for at least 3/4 weeks without giving in,but I would say it has worked for the majority of people I know have done it.
Good luck