If he is not drinking any colostrum directly from his dam you may need to tube in a bit more than 50ml every 4 hours to make sure he gets the correct amount depending on his weight. Found this guide which I hope helps -
'A lamb requires between 180-290ml of colostrum per kilogram of body weight, this means each lamb requires over a litre (based on an average birthing weight of 5-6kg) of colostrum. Colostrum requires a lot of investment, energy-wise by the ewe so if she gives birth to multiple offspring, it can be very demanding to keep up with the amount of colostrum required by her lambs. For maximum efficacy, new born lambs must consume the required amount of colostrum within the first 18 hours of life, putting even more pressure on the ewe.'