I expressed for 3m - the most miserable time of my life. Then he latched on!
Sometimes i think that if i had just caved in and given formula, he might not have been quite so healthy, but our relationship would have been better and I might have had at least one happy memory of his early babyhood.
But just in case, have you tried the nipple shield things? Also, my 'lactation consultant' (this was in the US) produced something called a periodontal syringe - like a medical syringe but it finishes in a short, curved, plastic tube (meant for cleaning gum crevices, i think). This I was able to fill with EBM, slide into the corner of DS's mouth while he was latched on but not sucking, and give him a squirt of milk as encouragement. It was this that did the trick - he soon got the message that nipple = milk and started to suck properly.
Just be sure to do the squirting at slightly irregular intervals or he'll just learn to take it for granted and wait for it. Good luck!