I'd bottle feed him twice a day. Maybe only half to two-thirds what you'd give an orphan lamb each time, and only twice a day, because mum must have some milk, just not enough. So he'll feed from her in between your two top ups, which will keep stimulating her production.
And I'd have him and mum out on the best grass you've got - she won't be able to produce milk without enough input. You might want to give her some concentrate too.
What is the ewe's condition like? Do you know how to condition score? Can you feel her spine? Her ribs? The bones in her tail?
When you see that the lamb's belly is already rounded when you go to give him his top-up, then you can start to reduce the top-ups.