Lambs of the age you have can be impossible to persuade to suck a teat , so try confining in a small pen with a pan or bowl of milk and a similar container with lamb creep plus top quality hay and cut grass fresh every day . Or you can just leave it to eat grass and drink water . In both situations lose of condition will happen ,its just how fast the lambs rumen develops . Tubing a big lamb is very difficult but if you wish to percivere then get at least half a litre in to the lamb or try using a dosing gun and slowly putting maybe 10 ml then stop then 10ml stop etc .