Em please don't feel that way! Your situation sounds identical to mine. Reuben was feeding well and often and his latch looked perfect from the outside and wasn't hurting so everyone assumed all was well. He lost weight at day 5 and day 10 and had only gained 20g on day 12. It turned out to be an undiagnosed tongue tie which was stopping him feeding effectively and because he'd been feeding that way for two weeks my supply had suffered. It was a vicious circle of him not feeding properly, which made my supply bad, so he hardly got any milk, and didn't have the energy to grow or gain weight. We're just starting to break the cycle now. You could try some of the things I've done...
You could get him checked for tongue tie. Lots of midwives aren't experts and can't always tell, so if I were you I would look up an expert to come take a look. You do have to pay but if it helps his feeding I'd say it's worth the money. Reuben had a posterior tongue tie of about 50% but every midwife that looked just said "well it doesn't look like a tongue tie" just because it wasn't a typical one.
Rent a double pump from NCT and start expressing. If his feeding isn't efficient your milk supply will suffer so it's best to counteract that sooner rather than later. Then you can give him the expressed milk as a top up and you don't have to use a bottle, you can cup feed or syringe feed once he's had boobie.
I'm taking more milk plus which is a bit pricey, but I was taking fenugreek tablets which is a bit cheaper. Both will increase your milk supply.
You don't have to take the formula route if you don't want to. I'm very upset with the way I've been fobbed off by the midwives in my area as the answer to everything for them seems to be "top up with formula" as I was told that over and over.
I'm now bottle feeding with expressed milk and formula when I haven't made enough. Once he's put on some weight I'm going to attempt to get him back on the boob but it may be that too much time has passed to really get him feeding successful and I will carry on expressing and bottle feeding.
My supple is increasing day by day so I'm hoping eventually I won't need formula top ups, that's the main aim. He's getting about 80% breast milk at the moment so I'm happy with that for now.
I've felt every emotion under the sun over the last week or so and it feels like I'm only just coming out the other side and emerging from this horrible dark cloud.
I'll tell you what my mum told me because it's absolutely true even though it's hard to believe at the time. You are doing your best, you are not failing him, and you are amazing for persevering when the easy option would be to give up. You're a fantastic mummy and want to give your boy the best.