You've obviously checked for stones, thorns etc so...
He could have stood on a stone and its a bit sore, he could have knocked it, twisted it playing/jumping around which as resulted in some residual pain. Metacam - its an anti inflammatory, used on all animals it seems - is what we use for our sheep - we've not used on lambs before though,so check whether it is suitable for the age. You get it from the vet; you may want to contact your vet sooner, before Tuesday, to get some (if suitable for the age) as waiting until the vet turns up on Tuesday is a bit too long really.
You may also want to bring lamb and mum + any other offspring, in to help rest it as if its knocked he may require a couple of nights in just ot take the weight off it and help healing.
Hope this is some help
CTC