Baby fighting sleep!

My son is nearly 4 months old. Over the last 4 weeks it’s been harder and harder to get him to fall asleep. To get him to nap during the day or to get him to sleep at night I have to walk about with him in my arms and sway, for 15 mins until he falls asleep. Sometimes it can take up to half an hour. Then I can put him down. I’m worried that he should be starting to fall asleep on his own now. Plus my back is killing me. I do have a baby carrier that he will fall asleep in. So I use that too. How do you get your little ones to fall asleep?