For the mattress I would create a simple cube, then stretch the sides out to the dimensions you want in Edit Mode, add a Sub-division Modifier set at 2 for render and view (also check “Optimal Display” checkbox), then add some edge loops (CTRL+R) and slide these near to the edges so you get the curves you want. You can then deform some the top faces to show a depression where the man lays, by lowering them in Z axis.
For the rig - you used Rigify to rig him right? just set Rigify to IK mode (In the left side menu 3D view) and drag the feet IK targets so the legs are bent as you want. I don’t have time today to post a blend file, so this will keep you going until I can spend a little time maybe tomorrow or Monday.
PS. There are no easy shortcuts in rigging and animation that will bring you good results!
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