Help!

My 3 (almost 4) month old son has recently learned how to roll from his back to his stomach (not vice versa yet). He’s not quite ready to not be swaddled as he constantly wakes himself up with flailing arms, but tonight he rolled to his belly in his sleep. I woke up to see him on his belly, and panicked given that his arms were swaddled, and rolled him back over and he woke up fussy until I nursed him back to sleep. 😬

I have an owlet monitor and those readings were perfectly normal while he was laying stomach down, because his head was turned enough for him to breath but I’m still nervous. Im using the halo sleep sack swaddle and wondering if now is the time to put at least one of his arms out, or if I should wedge a light blanket just under his left shoulder/back out of the way of his head/face/neck to prevent him from rolling to that side and onto his tummy.🙃

Any advice is appreciated for this transitional time. We do not co-sleep, he sleeps in the Halo bassinest next to my bed.