Hi Janos, you are doing really well, expressing is so hard.
My baby, although he wasn't prem was very small and lost weight after he was born (down to 4lb 10oz) and he refused to latch on for the first month.
It was only with the support and advice of a couple of bfc's that I was able to carry on and now he is still breastfeeding at 9 months old.
Breastfeeding counsellors are all volunteer mums working from home so not all can visit you at home but I would advise you to contact all the available groups to see if there is anyone near to you who can.
One bfc suggested bringing ds up from underneath with me sitting on the edge of a chair so in efefct he was horizontal with me supporting his bottom. She is also a hospital midwife and used that technique with prem babies and it was the first time he was able to latch on (though he had still to learn how to suck!)
You may have been given this advise already but just in case I will say it. Spend a lot of time in skin to skin contact. Undress your baby except for a nappy and put him against your chest, take him in the bath with you. try expressing a small amout of milk and dripping it down your breast from a syringe before trying to latch him on so he gets used to the idea that is where the milk comes from.
Also consider (apologies if you are already doing this) using a special infant feeding cup rather than a bottle so that he does not learn the wrong technique.
Some babies just need to grow a little, it is possible, it probably feels like you are fighting a lost cause. You are not, hopefully your ds will start to feed preoperly soon but you do need lots of support.
Try to rest as much as possible, get someone else if you can to do all the sterilising etc and remember that you only have to sterilise the pump once a day. You will also finf it a lot easier if you are using a double electric pump. The NCT hire them out.
Hang in there, it does get better.