Assuming the lamb can still hold its head up, it needs as much colostrum as you can get into it, urgently. When you say 'sachet' is that sachet of replacement milk or of colostrum replacer?
I always aim for a minimum of 100ml colostrum, in two feeds if necessary, within ideally the first 2 hours and at most the first 6 hours. More is better, up to half a litre, but my experience is that lambs which get less than 100ml colostrum or colostrum replacer within the first six hours will die.
If you haven't got colostrum replacer and can't milk the ewe, do you have a dairy farm nearby? The farmer there may be able to let you have some cattle colostrum - it's very much third best but it is better than nothing.
It sounds as though you have got the lamb sucking the bottle now - well done. But if it won't take as much as it needs you may still have to tube it.
Oh, you've just posted again and you think there is a chance it has taken colostrum from mum. Let's hope so. 50-100ml extra certainly won't hurt it - but I wouldn't overfill it with formula if you think it can, or you can get it to, get mum's milk.
If the ears are cold and its lethargic, as Juliet says it could probably do with some glucose. I use Rehydion - it's actually formulated for calves but I find it excellent for lambs. It gives energy, vitamins, and rehydrates. You can mix it in water if the lamb is very weak and in milk if the lamb will take it. Failing that, there's PSF (Pfizer Scour Formula) - again, designed for calves but works for lambs, which being a powder you mix with water will also help rehydrate. You can also use Lamb Boost or other similar squirt-in-the-mouth vitamin-and-energy products, but that won't help get water into it.
I don't dispute what others here are saying about glucose directly but haven't any experience of that myself.
Its immunity will be compromised by the lack of colostrum in the first few hours, so you may want to think about OraJet as a precaution against some of the bugs it will come across if it survives.
If the teat is huge it could be too big for the lamb's mouth; this can happen especially when the lamb continues to not suck and the udder and teat just get bigger and bigger and bigger. If this is the case and the lamb neither sucks nor do you milk the ewe out, the ewe could develop mastitis and/or metabolic disturbances.
Can you not tip the ewe over on her bottom and milk her into a bottle or jug? If not, get a halter on her head, tie her up, and use your body to hold her against the side of the pen while you milk her into a jug. If you can't, get someone - anyone, although someone experienced, a farmer or vet would of course be preferable - to help you.
Re-reading your post it sounds as though the ewe and lamb are running around outside? I don't know where you are but if it's winter there I would have them penned up in a lambing pen in the warm and dry where I can check them every couple of hours and keep topping the lamb up / hold it against the teat if needed. If the lamb is weak and not sure of the teat, being out in the open it is unlikely to get enough milk.
And as the lamb has cold ears, although thankfully is not cold in its mouth, it sounds as though somewhere warm and dry for the night for it and mum would be a good plan.
I'm sorry this is disjointed - I'm trying to adapt it as new posts come in!
