Please talk to your midwife - you can tell her that you're afraid she'll send you back, and I would really hope she'd talk to you about feeding and your babies weight. If it does become necessary for you to go back into hospital, that should be a joint decision between you and your midwife and only done for the good of your tiny baby.
Your milk probably hasn't come in yet, though it will over the next few days. In my experience, that makes feeding much easier because the baby starts swallowing more (because there is more to swallow) and that helps them stay latched on. The best way to encourage your milk is to be close as close with your baby, preferably skin to skin if your house can be made warm enough for that to be comfortable.
A good way to encourage feeding is to lie on your bed propped up a bit on pillows. Put baby (in just a nappy) on your bare tummy and cradled by your hands and let him sleep and snuggle there. When he gets hungry, he'll probably wiggle up your body to find your boob - I didn't believe that could happen, but my DS did it! It took him a few minutes (and it hard to resist 'helping' him), but he got there and latched on. Then of course, he fell asleep and fell off the boob again, but the goal of stimulating both you and him will have been reached :)
Try not to panic that baby is hungry all the time. Babies (especially new ones) cry for all sorts of reasons, mostly because they just don't want you to put them down (and evolution has made it virtually impossible for a new mother to ignore a newborn's howls). He just want you, your warmth, your skin, you smell, your taste. If you can warm up your bedroom, go to bed with comfortable pj's or a naked top half, tuck the duvet around your hips and lie with your baby next to you. You might or might not manage sleep, but it's so much calmer there in your bed than anywhere else, so it'll be peaceful for you both and will let you snuggle in peace. If you're worried about baby falling off or you rolling onto him, tell your DP that he has to sit in the bedroom too (with book/laptop/all the black socks that need pairing?!) to keep an eye on you both if you dose off. Nothing else matters - make time stand still for a few days.
If you're in North Cambridgeshire I can recommend a great Breastfeeding group run by the worlds best breastfeeding- friendly midwife, and a breastfeeding peer support network of lovely supportive mums too. Your area will almost certainly have these things too - ask your midwife for the name and number of the hospital's Infant Feeding Coordinator and she should be able to get you the details of these types of support.
Lastly - and sorry for the essay - go easy on yourself. You've just been through a serious physical ordeal, you've had no sleep for days, and you're adjusting to being a mum. None of those things are easy, and unfortunately they all come at once
. Talk to your midwife, ask for support, lean on DP and try to stay in bed!
Take care of yourself ^