So far have given him 2 tube feeds, 2 days apart. Today he seems much better, lively and strong again. I now have high hopes that I can at least can keep him alive, the issue will be how to get him back onto solid food, but that's one for the future.
For anyone who might be in this position, what I'm doing is taking a quite solid oily paste and suspending it in bottled water, then giving about 1ml of this very dilute suspension through a flexible plastic pipette (dropper). I think that the water is as important as the food, as he was probably very dehydrated as well as undernourished.
I'm currently giving this at 2 day intervals. If he improves on this I will start to reduce the dilution and give less frequently. If it works I'll maybe start a separate thread on the process.
Anyway, thanks to all following.