All the other suggestions are great but I have just one more......

Now this might sound silly but stop spending alot of time trying to get hiom to sleep.. Sit down with him cuddle and let him rest on you.. Yes I know you have other things to do but the more time you spend trying to get him to sleep the less he will sleep... I went through something similar with my 3rd son and I realised after a couple weeks that I needed to take his cues and just relax about it all.. I also left him to cry if I was in desperate need to do something such as cooking dinner or tending to my other children.. I hate hearing him crying but I didn't have any other choice and after a few days he realsied mummy would pick him up if she was able and I don't know things just worked out..
I hope you can have a similar sort of result as it makes life so much easier...
((hugs))