Sleep recommendations

Okay, so my baby is almost 4 months old and still like to sleep in a halo sleep sack swaddle. The problem is that yesterday he mastered rolling from his back to his belly and now I do t feel comfortable trapping his arms while he sleeps like that. We tried sleeping without the swaddle last night and that was a disaster. He was flailing his arms, startling himself or hitting himself in the face all night long. 
Is there a product that I can use in his situation? (I've heard of the magic Merlin suit but was unsure if it would work) What suggestions do you have for getting your baby to sleep without the swaddle!? Any help is appreciated. Thank you!!