rnbsmum - what a palaver.....on the other hand, I would be reluctant to suggest you stopped doing what you have been advised to do without suggesting you asked the mw what she thought first.
The best way to increase milk supply is to remove the milk frequently and effectively. The only way to increase the milk intake of your baby is to get the milk inside him....now, some babies who don't gain weight might have mums whose breastmilk supply has drifted downwards, and their lack of weight gain makes them too tired to persist with long 'switch' feeds at the breast (when you go back to the first side again and then the second again). I think if your baby has remained at 10 per cent under birthweight for 4 weeks then this scenario may well apply, and it may well make sense to give him expressed milk after the feed at the breast ... I don't know enough about your situation to know whether he needs top ups of formula after this as well.
If you can increase the frequency of his breastfeeds, this will be the most important bit of helping him gain weight, and don't be afraid to put him back again to the first side when he stops feeding on the second.
Important to use breast compression (google it or look on mumsnet) as this does encourage a non-sucking baby to start sucking again. If anyone is trying to build up a milk supply, they have to think of the minimum no. of feeds in 24 hours as being 8-10, and the night feeds are v. important.
Yes, express as well, so you inc. the stimulation and also you have a back-up if you want to increase his intake and he doesn't want to go back on the breast or if he is not sucking well.
Hope this helps.