Sounds like you are doing the right things.
You’ll know, I’m sure, that the chances of his making a full recovery and reaching adulthood are not great, after his poor start. But some of them do survive, and if he had enough of his mum’s colostrum before he got lost, he may well pull through. Be aware though, that many such grow well for some weeks, then founder after a couple of months. Pointless to tell you not to get too attached, I know.
Assuming he rallies from this and starts to be a typical bouncy healthy lamb, then as well as milk as per the bag, he will need :
- a little fresh hay every day to chew on, essential for rumen development
- company of his own kind; once you’re sure he’s pulled through, best get a pair of orphan lambs, then if any one dies the others still have company
- after three weeks of age, vaccinations.
You’ve got some land? Are you thinking of growing him/them on yourself or will you need to find a farmer to take him/them?