Dont know if anyone can answer this for me

but when I get back from work in the evening and in the morning I have noticed that 1 lamb is on his own, he stands in the top corner of the field, staring at the fence with his head and ears down baaing quietly to himself

he is also sleeping away from the others curled up on a patch of mud (reseeded some of the field). so he is alone and very dirty and looks awful. He is also thinner than the others now.
I have started feeding the ewes some sugarbeet before rammy comes and all the others come running, but he doesnt even react, I can walk up to him and pick him up and I have noticed he has a bad foot (shelley hoof I think) but I am worried about him now
Anyone have any ideas? I really cant afford the vet and unfortunately he isnt really worth it what with the vet charging £42 call out charge now
thanks
jess
Why would a sheep start isolating himself?