Umm... I think a Welsh Mountain is likely to need considerably less feed than a Ryeland

- so if he's taking more than 500ml in 24 hours, I wouldn't be too worried.
My waifs get 6 feeds for the first day or two, then they're onto 4. Three hourly is unnecessary, in my view.
If you let him get hungry, he'll drink alright!

That's not as heartless as it sounds - truly, feeding too often makes them only want to take a little at a time (which is the natural way, they'd take little and often off mum if left to their own devices), so as he's to be bottle-reared I'd be getting him used to fewer slightly larger feeds straight away - as soon as you have him recovered from any ordeal he's been through. (And yes, something like Co-late or Kick Start is a real boon in those circs.)
Make sure he has good clean fresh hay available, and clean fresh water to drink, so he can nibble and sip on between feeds. It's very important they get long-stemmed forage in the first seven days for rumen development.
The other thing that could be going on, if you really think he isn't taking enough milk to keep him going, is that his system was overloaded by that first 5oz feed and now he's feeling bloated and that he doesn't really like that bottled milk stuff. If you think there could be an element of that, get some Pfizer Scour Formula, or Rehydion (they're both for calves but perfectly good for lambs) and put him on that for 24 or even 48 hours. That'll give him vitamins and electrolytes and let his abused digestion recover! Then back onto milk - with Rehydion you can mix it with milk so can reintroduce milk slowly, with PFS I'm not sure mixing is a good idea, so you could replace every third feed with a small milk feed on the second day, then hopefully he'd be okay with all milk by the third.