Sounds like mum may have rolled or trod on his trotter, have a feel i you can, if he is in pain it may be broken.
Now if it is you have two options, blunt but straightforward knock him on the head and cut your losses. BUT have been down this road myself and we had a runt similar to yours called Tiny, i wrapped him a bath towel and OH took us both down the vets where they put a little pot on his leg, he soon got the hang of it and was back feeding off his mum and hobbling about witht the other piglets. This + antibiotics (he had cut as well) cost me £90+. So its up to you you pays your money and takes your choice. You may find that if he isn't feeding he'll go away to a corner and just curl up and die which isn't very nice. So if you think theres little chance of him making it i would suggest PTS by the vet if you can't bear to do it yourself. OH dispatches our still breathing squashed, injured tinies defo couldn't do it myself.
Sorry can't be more help.
Mandy

Ps Tiny made it to pork weight and was duly dispatched with his brothers!