k...by kate's request...
both my boys have/had reflux. I am unfortunately well versed in swaddling, incline greater than 45 degrees for the mattress, wiggling, getting out of the swaddle, medicating, etc.... So, my experience has taught me how to make a good little "nook" so that your lo can't wriggle down the mattress and you can put him on very steep incline too.
Use rolled up receiving blankets to make a square (but with one end open...so really a "U") I rolled two receiving blankets together for each side (6 in total) then placed a blanket (like their crib quilt) on top so that it made a nice little "nest" then settled their little bums against the bottom edge so that they were good and snuggled feeling with the two sides butted up against them. The roll under the bum helped to keep them in place (especially when a blanket is placed over the whole thing....then the rolls can't roll away IYKWIM??). HTH!
It gets better every day.
also...since your lo likes his legs up the roll under the bum should be extra comfy for him.