Maybe try a lamb adoptor (google it)? They're cheap and available from your local fencing man. Keeps her in place (with a nice bucket of treats in front) while small boy nuzzles in - maybe if she realises there are treats in place while she does it ...? you could perhaps reduce the treats as the week goes on and she gets in the groove of things.
You may have to check for mastitis if she overloads with milk or if any of her teats are sore etc.
Is it possible you could also block her in with him by use of hay bales 2 high all around them?
I'd be tempted to keep them away from the other mums, she's being lazy. All the same, he is getting what he needs one way or the other and as a boy he is likely to be on the solids quicker than females.