evansmummy, efficient nursers certainly exist - I've had one efficient one and one inefficient one. The fact he's gaining fine means the problem is the hassle, which will probably settle down over time, and isn't worth getting stressed about, if you can help it.
If the problem is re-latching on - if he wants to latch on, but can't seem to, you can help him by ... how to explain this ... You can grab your breast, well behind the nipple, with forefinger on one side, thumb on the other, and squeeze a bit, so the nipple acts a bit more engorged. (If this hurts, or doesn't work for you, don't bother!)
I'm not into osteopathy, so don't know what to say there.
Could he be tired? How are his sleep patterns?
Is he being distracted by things? I've found DS2 started getting distracted pretty early.
Given he's gaining fine, can you give up quicker? If he pulls off and doesn't want more, then he's done. Why cause everyone stress?