This is just my advice natalie and you can take it or leave it:

I am not a big fan of putting them down and just letting them cry - it goes against my instincts. I breastfed to sleep day and night, and for me it was the fastest, easiest, no-cry solution to getting Charlie to bed. The less I stressed about him self-settling, the easier it was for both of us, and despite all the gloomy warnings from all and sundry that he would never learn to self-settle, he did learn all on his own. He now goes to bed after his bedtime routine of milk, bath, brushing of his teeth and kissing everyone goodnight, and I leave him in his room awake, and he will put himself to sleep.

5 weeks is really young to be trying to get them to self-settle. Like most things, kids usually learn those skills in their own time, and it may end up being more stressful for you and for your DS in the long by trying to 'train' him to do something he might not be ready to do yet. He probably sleeps well at night because going to sleep for him was a pleasant, easy experience, falling asleep on mummy's chest.

Keep in mind also that it's normal for little ones to not sleep more than 20 min blocks sometimes, so it might be easier to get him to sleep in a way that's convenient and pleasant for you both, so that you can get some rest before he wakes again .