I swear this is a higher beings way of telling us as parents that we're done having babies, when we get that one AWFUL sleeper ;)
Just remember that this can't possibly last forever. I have no advice other than remembering that! ... my 3rd child, my only son, is still a crappy sleeper... no more babies!! I couldn't go through this again...
Also, if he has his days and his nights mixed up, make sure you're doing your best to keep him awake and alert during the day. MIGHT help, or might make him overtired at night. Worth a try...
Hope it gets better soon :) Just remember, what goes around comes around, and in 20+ years that baby of yours will have his own baby, and be calling you crying about it!! ;)