It could be because of his frequent feeds? I was told this today after my DS is showing signs of reflux. Can you try to space out his feeds to just three hourly (or even 2.5 hours if you just can't stretch anymore)? You could try wear him in a sling or take him for a walk in his pram to help stretch him out. I'd also try Mickyndan's suggestion about raising his cot mattress so his head is higher than his body (I use towels under my DS's mattress).

Hope you get some answers soon