My little guy outgrew his swaddle sacks as well. Now I put in in a sleep sack and wrap his arms down with a receiving blanket. Here is a video demonstrating it. https://www.youtube.com/watch?v=4JiHCWEF-GI
What I like about this also is that if he happens to roll over in the night, his arms can get free as the blanket will no longer be pinned underneath him, but if he lays on his back, his body weight keeps his arms locked at his sides.