It is most vital that a lamb gets colostrum, preferably from its dam withing first 6 hours. After that it will have the incentive and ump to look for more and a dilute amount of packet stuff will not hurt. My weeny lamb is still weeny but very robust now after being the weakest tinyest lamb I have ever managed to keep alive. Remember a ouessant will need a minute top up...no more than 20-25ml at a time!
There is one problem I forsee.....if you leave the lamb with his mum and he is feeding from her he will not take a bottle so readily....or he might, but that will eventually leave his mum with too much milk and then she may well get mastitis.....It is very difficult to leave a lamb on a good mum without her producing milk for him....and trying to bottle feed.
In my long experience it only works in a ewe who loves her lamb but has a non functioning udder...i.e one that lambs and you find to your dismay that both sides are damaged from previous mastitis and this is usually in a ewe that has been sold to you be someone who 'knew' .
In cases like that its very possible, but with a ewe with a functioning udder, and esp one who is underweight and whom you want to feed up! All you will do by feeding her up will make her make more milk!
I know this sounds hard but have you considered waiting a few days, getting him on a bottle, and then having his dam pts if you are not going to breed from her again and you think she has an underlying health problem?
Can you post a pic of her so we can see exactly how thin you think she is?....just a thought