My
DS was never a good sleeper but I do remember that his sleep deteriorated at about 5 months. I hear that it's quite common - between all the crawling, starting food and other developmental leaps, it must be hard for their little brains to wind down at night.
I also had 2 consults at sleep school, the NCSS book (and about 10 others) and very quickly found that it was a lot easier to just go with it - some things work and sometimes nothing works. Just in the last week
DS's night sleep has improved and he's 1 now. (Hopefully it's not a fluke

)
Also you might want to try staying in
DS's bedroom if he wakes at night, even for feeds. I always kept the lights low, no playing etc at night so
DS knew it was sleep time. It sounds like you're probably doing all that anyway, but just a thought.
Good luck Salsa - our bubs will grow out of all of these difficult stages. We just have to hang in there - it's so hard when you're tired and just want sleep!!!
Hope someone else might have some advice for you