hi sparkly.
firstly, sorry you are having such a terrible time still the stress cannot be helping at all.
I'm sure this has been said already by the wise mums on here, but I was going to suggest feeding more regularly than 3 hours.
Heres my experience: My dd was little and prem. she went down to under 4.5 lbs with a huge weight loss when she was still in NNU.
I was feeding 3 hourly, then on demand. Her 'demand' was not enough, so i expressed very regularly, and fed her every 2 hours, then topped her up via NGT. I just fed from one side at a time, and expressed from the other post feed. It DID mean i was almost constantly doing it, and I know you have other children, but it may be the only way for a little while...
Isobel started gaining weight, slowly, but adequately. Have you considered breast feeding him, 2 hourly and topping him up with formula towards the end of the feed, or just after via his tube? I know you dont want to FF, but this way you are still bf, every feed... and they may back off if he gets a boost in weight... and then you cold stop?
Just an idea, I know not what you want, but maybe a little compromise just for a week or two. YOu will also not harm your milk supply if you still feed this reguarly, and if he doesnt feed for long, then express as well.
It was the only way i could get her gaining in the NNU. I exclusively bf until nearly 6 months and have no intention of stopping yet, and she is a gorgeous 10 month old girl. She is still on the small side, but sitting on the same point on those bloody development charts. She is eating well now, solids and breast feeding.
irony being, she wont take formula nor will she take a bottle even with EBM and at times, i'd love to be able to have a day to rest ( have probs with pain) and I cant give her to mum for the day! I was so desperate that i would bf her, and had such a tricky start, i cannot bare to think of not doing it now!
Hope some of that helps, sorry it went on so long!
good luck.
ps I am also a NNU nurse, when not on adult ward... oh, well, when i was working just before isobel was born!