D.W.
My son was born at 32 weeks exactly and he was in the NI for 6 weeks. But I was able to teach him how to nurse once we got home. It took about 6 weeks but we then went on to nurse till he was 16 months old!
I first switched him to the playtex nurser with the latex (brown) nipple with the drop ins. The latex nipple is very very soft like the breast and with the drop ins you can squirt just a tiny bit of warm breastmilk in his mouth so he understands why this new nipple is in his mouth. I would also feed him with my shirt and bra off and lots of skin contact, would burp him then change sides just as if he were nursing. Once he got good with that we started adding in nursing sessions. And he got it! Since your son is older it might go quicker than when we started at 6 weeks old with my son.
Also, make sure you are using a slow flow nipple. Nursing is more work for baby than bottle feeding so you never want to use a fast flow nipple.
I pumped round the clock for a solid 12 weeks until I taught him how to nurse and was able to build a good supply. Make sure you are using a high quality (or even hospital grade) double electric pump. That made a huge difference for me.