Oh golly, you must be feeling desperate about now (I can remember it!!).
First things first, this will pass. My 5 month old DS slept terribly from 6-13 weeks, but it did come to an end eventually, and he sleeps okay now. I think you will find that something similar happens with your little one.
Could it be colic? Have you tried infacol if you think this might be the case.
I also found that baby massage helped DS to settle - check for local classes, sometimes the children's centres run them for free.
The happiest baby on the block techniques also helped our DS - swaddling was great for getting him to sleep. White noise (we recorded the tumble drier and played it on a loop).
Try getting him to take a dummy (our ds wouldn't).
Get your family to help out. MIL was a life saver and stayed over a few times to take a shift. She loved all that cuddling time!
Get a carrier or sling that you and he both feel comfortable in. We love our babybjorn, but others swear by slings.
Oh heck, just spotted you have a toddler too! Hope one or more of these helps.