Hi Mandy
How much is he getting per feed?
Is there any chance of him spending days with his mum and twin?
Maybe lambs come on quicker than goats but at 3 weeks I'd still have them on 4 feeds a day, but less per feed, in fact I've some kids 9 wks old that still get 4 feeds a day,
My sheep like rosebay willowherb, maybe try him with some stems of that? Hung upside down in his pen?