Because you described the wrinkles, and the "empty" belly, I assume the vet gave you instructions how to notice hungry babies.
It is common in rabbits that the mum feeds her babies only once, seldom up to three times per day. She stays away from the nest to not attract predators and secure the safety of the babies. SO you need not to worry, if you notice this behavior.
First try: Maybe you can help the mum to give milk to the babies.
You can control the weight of the baby once per day, and see if it stays the same or increases. For this, you need to distract the mum in a way that she will not notice the weight being measured. Use food and let her roam and block the way back. Make the weight measurements as short as possible, wash your hands and dry them well before.
Before thinking about hand-feeding the baby, you need to know that hand-fed babies have a higher risk to die and also a higher risk to get disease in their later life. So your first goal is to get the mum to feed her child.
As long as the baby gains one gram per 24 hours, one should not feed with hand.
The mum also can get the milk a little late after birth. Suckling babies would stimulate the mum's body to produce (more) milk.
A second option is this: two times a day, the baby and the mum will be set together into a small box, or the nesting area will be fenced, so the mum needs to stay with the baby for around one hour. This works in some cases, to stimulate the mum to feed her baby.
You can (exceptionally!) turn the mum and hold her with the belly up, and lay the baby onto her nipples. You need to stay as calm as possible, to tell the mum that all is okay, and this is not a dangerous situation. Naturally, the mum may not like it much to be turned. As long as she does not move wildly to get free, you can try.
Here you can see a short video of www.kaninchenwiese.de, how this looks like. It is easier if you have more than one person, and if you keep the eyes of the mum shut with a towel or your hands.
What causes problems in feeding?
Not enough peace for the mum. The mum need to be in her known environment; all should be as she is common with.
Controlling the nest too much. The control should be only once per day; a short look, as quick as possible and without attracting her attention.
Not enough space. The mum has the instinct to stay away from the nest. If she cannot make enough space between her and the baby, she will become stressed because she feels permanent danger for her child. She will become nervous and has not enough peace to sit and feed.
Conflict with other rabbits. If the group is not harmonic enough, other rabbits can distract the mum from feeding.
Not enough food of good quality. Rabbits (especially mums) need much fresh food (which means not dried; leaves, vegetables, etc. as much as she wants to eat, so if she does not leave any, give her more!) and additional high energy food, like seeds (no grains), 1 soup-spoon per day (have a look in this question, for useful seeds and one option to feed them: What is Rabbit Mash and how is it useful for challenged rabbits?).
The nest is dirty, maybe stinky. Please clean it (bloody/wet straw, leftovers of the birth).
The nest is not fitting the mum's and baby's needs. Build them a nice cave, the mum feels her baby safe there.
Please do not feed the baby cow-milk, or condensed/evaporated milk!
IF the steps above do not work, then you need to buy special milk-powder for babies of rabbits or goats/sheep (in need you can use some powder - not liquid milk - for cats too) and mix them with "Rodicare akut" or anise-fennel-caraway tea (there are available for human babies and in the health part of big supermarkets or in the pharmacy) and oil from lineseed or rapeseed to get the needed high fat contents. The resulting milk should have 12% protein and 14% fat (so you need to adapt the recipe of the powder). If the baby gets too much gas in the belly, you can add a drop of Sab Simplex; however, do not add it preventively.
Hand feeding needs to be more frequent than the mum would feed, because the exchange food is not the same high quality as the mum's milk. So feeding should be done once per 4-7 hours, depending of the health and age of the baby (more often in small, weak rabbits).
To feed, one can use special bottles, made for small animals' babies, or a 1 ml syringe (without needle of course). The baby would lay on its back when suckling in nature, but one needs to be very careful that no milk gets into the nose. Feed slowly in small amounts (maybe you want to test/train it without the baby and with water first). If milk gets into the nose, it gets into the lungs and the baby will have a high risk for pneumonia and death.
How much milk should be given per meal is not a uniform opinion. But it should be 1 ml or more, depending of the hunger and age of the animals.
In the beginning, it could occur that the baby will not suckle the syringe/bottle itself. Then, one can enter the mouth careful from the side and give one drop into it. The babies learn fast where the food comes from. After finishing, the mouth should be cleaned with a clean damp cloth.
(All information is translated from this website: https://www.kaninchenwiese.de/nachwuchs/paarung-geburt-und-aufzucht/handaufzucht/)
