i was also going to suggest the 'love nest' pillow - looks a bit weird but has helped my DS too...
i took it to my nurse and she said it woudl be ok fro a SIDS perspective too... (i was worried abot that)
aside from that i was told to keep him off his back as much as poss when he is awake - so i dont use the pram basinette anymore (he didnt like it), i do lots of tumy time, he sits in bouncer/swing, carry him in sling... i was also told it will resolve and get better as he gets older and sits up more etc. it has improved a lot already...