Okay so I'm completely at my wits end
My 12 week old is formula fed and since about 8 weeks it's been impossible to try and feed him — he just refuses, screams, cries and his feeds are all over the place..
Sometimes he goes 6-7 hours without a feed and then will only take about 4-5 oz, I offer him the bottle all the time and he will just refuse and refuse until he is truly hungry
Sometimes he'll only have about 20-25 oz a day and that's a good day :( he's supposed to be on about 35-40 according to his weight
He doesn't wake for a night feed and will refuse if I try and wake him — he sleeps completely through and then sometimes isn't even hungry when he wakes up
The worst thing is, he's had a case of diarrhoea for about nearly three weeks now, we've been to get him weighed and he's lost quite a bit of weight
We've also been to see the doctor who simply said we're underfeeding him?? I know this but it's not like I can force I down his throat!
They've taken samples for milk allergies etc but said it's unlikely since it only started about 8 weeks — it isn't reflux/colic as he doesn't seem in pain, just throws a massive tantrum when I try and offer him the bottle! 
Anyway I'm just hoping for some words of support or similar cases, anything you think may be wrong. Every feed now is reducing me to tears because it's just so frustrating and I don't feel like I'm doing a good enough job :(