I dont really have any suggestions but just wanted to say that Hendrix was a fabulous sleeper until the 4th month and then went through exactly as you describe. I suppose my only real advice is to stick to the routine and maybe do a top up feed?? I would do a feed at 4 and another at 6 to tank him up as he was going through a growth spurt as well as growing in leaps and bounds developmentally.
Another thing is that understand that it will pass in time (im sure your sick of people saying that, but it will )
HTH