Gosh massive hugs (which sounds trite under the circumstances I know), I can't even begin to imagine how traumatic things have been for you - but what an amazing inspiration of a momma you are!
You mention DS started out on the 91st and is now below the 25th - can you expand any on his growth pattern? Did he downsize and drop rapidly down a few centiles after birth, or has it been a gradual drop? what sort of figure has he been gaining on average? Probably most importantly, are you plotting him on the new exclusively breastfed chart?
A good question to ask yourself is, if you hadn't had him weighed would you be concerned about his growth or how much he was taking? What is his length like ie does he look in proportion or does he look to be getting slimmer? What was the growth of your eldest like what does your gut feeling tell you?
Sleep cycles also change around 3/4 months and lots of infants do experience waking when they have slept brilliantly before - this may be related to intake, but is very frequently completely unrelated and in fact far more about developmental growth. Is he trying to master any new skills such as rolling or sitting up, any drooling or signs of teeth moving about under the surface?
I think as you say maximising sleep for you is important - where does baby sleep now? if separate maybe consider converting the cot into a side car cot so you can feed without really waking/getting up? Several studies show sleeping next to baby without a barrier results in not only more sleep for mum, but also better quality sleep - leaving mums feeling more rested (a recent study showed different sleep phases were hit for different lengths of time, with a longer period of the most restful type)
You could try increasing intake during the day too, to see if it helps - which would also be relevant if you do feel there is genuinely a weight issue.
There's a few ways you could do this - if you look at the bottom of this article (a lot wont be relevant to you), under the header "tips for increasing gain" you should find the info you need.
www.analyticalarmadillo.co.uk/2010/09/is-my-breastfed-baby-getting-enough.html
HTH a bit :) Sorry for the rather long reply.
AA