Hello Lou and congratulations on your little boy, it sounds like he is doing really well. My dd is an ex 27 weeker, now 2 and 9 months. She was bf from birth with EBM and I still do not know how I got throught the first 3 months at home.
If your nipples are sore and cracked, something is not right no matter what anyone who has checked the latch and position at the breast says. As well as having his mouth wide open and sucking well, does he have a good mouthful of breast? As others have said, try and see a breastfeeding counsellor or find a support group where someone can watch you feed and offer constructive help.
You can consider using shields but they will not correct the underlying problem. However, I know a few mums with prems who found that their babies fed better with them until they grew a bit bigger.
You don't say what weight your ds is, but I am guessing somewhere around the 5-6lb mark? I can understand why you are being encouraged to think about formula top-ups, but I agree it's often done without exploring other options.
Re your supply, it is unlikely that your milk is drying up - there does come a point where your supply and your baby's needs are in line, although growth spurts can disrupt that and it's possible that your son is having one- I think my dd had her first about 2 weeks after her due date.
Take care and keep posting