My LO is 5 mths and he still startled at 4 mths old but now I let him sleep on his sides with a small pillow on his back to support and rotate the sides. Just make sure nothing else is on his bed
He will take time to adjust without swaddle, mine i swaddle my daughter until 3 months than stopped swaddling totally less than 2 or 3 days she got used to it.