Talk to the vet.
Could try Calciject or twin lamb drench if you have either around from lambing - both will raise blood sugar levels and neither will do any harm. Some tups just don't eat when around the girls, and this year's lamb won't have much 'reserve'. If that's all it is, he'll be back on his feet pretty quickly.
But it could also be much more serious than that - so phone the vet.