morning belinda, sorry you're having a tricky time. day 5 is just the worst in my experience.
a couple of things - first, 9% of birthweight is within normal, healthy limits for a newborn breastfed baby so there is no indication to reweigh, and I would argue that is causing more problems than it's solving. Is your baby weeing plenty? You say that poos are infrequent - how infrequent? is he very jaundiced? (some is normal). Was he a healthy, term baby? If yes and not jaundiced then your midwives are being overly anxious and unhelpful, and you could argue that he doesn't need to be reweighed because he is within normal limits as per WHO and NICE. and then tell them to bugger off. I'm finding that midwives are intervening at 9% more and more, when it's normal.
When did your milk come in? if it was only a day ago it may be taking him a while to catch up - there is a big difference between milk coming in on day 2 and day 5 where both those babies are weighed on day 5, hence the 10 % guideline, which you baby is, and i'm not sure if I've said this, WITHIN.
2 hourly feeds day and night is a lot for a woman with sore nipples, where there doesn't seem to be good cause. I would go back to demand feeding. because your baby is normal, and within normal ranges. did I mention that?
re the pain - if you think you've got thrush do get it treated as it will eventually go into the breast and cause horrendous pain or get to the babies mouth and then transfer to the other side. If it is the poor attachment then it may be painful still where there is trauma but should be less painful if the attachment has been fixed. Is the pain during or after a feed? thrush pain usually starts during and continues after a feed.
Has he been checked for tongue tie?
Is your milk supply good? when you do express and give him the ebm is he satisfied?
When he feeds do you hear long sucks and swallowing of the milk?
Are you doing lots (and i mean looooots) of skin to skin? if a baby won't feed from one side then continuous skin to skin can really help encourage them, as well as improving milk supply. I understand it's your 2nd child so you may not have the luxury of lounging around with your top off but if you have the support to enable you to do it then it can be very helpful.
Finally, if you do think your baby needs more milk (and if 9% loss is the only reason then I would argue that he doesn't - did I say that? :-) ) then he needs more breastmilk, not formula. Under no circumstances agree to give your baby formula unless a) you want to or b) he is very poorly and you cannot provide enough breastmilk. I consider the latter to be a very very rare case so unlikely to apply in your situation.
I consider over weighing of babies to be harmful, personally. I'm not sure if that came across. 
nb I am a midwife and a breastfeeder.
HTH.