HEY YOU come on and play with meeee! 
Here one of the lambs is exhibiting a classic sheepie gesture and "hoof-tapping" the other - sheep do this when they want something. Ours often do it to me when they want cuddles, fuss or more biscuits.
In this case the more energetic lamb is trying to get the quieter one to "get up and PLAY dammit!" }:-). And as it turns out being hassled to play like that is probably a good thing. We took the quieter lamb to the vets for a checkup today and it seems there isn't that much really wrong with him. The vet basically said "his legs are too long!". What that means is that his tendons are a bit overstretched, which makes it hard for him to put his hooves down straight. This is what has given him some slight difficulty in walking and held him back from being as active as his sibling.
However given time and exercise he'll grow naturally into his long legs and become normal, and the more he uses his legs the quicker that will happen. So being hassled by the other lamb to play is probably a good thing.
In the meantime we've been doing exactly the right thing in helping him to compensate for not being quite as mobile, by keeping his mum nearby and assisting him in getting some good solid nipple-time in a couple of times a day.
So good news really, all he needs is to carry on eating well and to get a little exercise and he should be fine. Which is a relief }:-).
Oh, and this morning while I was sitting cuddling him, his "sister" was using me for a climbing-frame, and I noticed something we'd both missed... either she's taken to carrying a small hairy purse containing a couple of marbles, or we were wrong about it being his sister }:-D.