In bed during independent play

Abigail

I try to get him to play by himself in 15 minute increments,4 times a day. I put him in his room and tell him mommy will be back in 15 minutes. Then I leave and watch him on the monitor. He climbs right in bed and covers himself up and lays there till I come back in. Any ideas how I can get him to actually play? All his toys are in there and I usually play with him for a few minutes before I leave. It makes me sad that he doesn't play and just lays there waiting for me. Sometimes he gets pissed but not usually.