I wouldn't be too worried about it. My older son didn't eat much in the way of solids until he was 18 months and my supply dwindled because I got pregnant again. Then when his brother was born and my milk came in, he stopped eating solids again, but only temporarily. At almost 3, he still nurses and doesn't eat much, but I don't worry about it.
I did/am doing BLW with both of my boys and it's so much easier than spoon-feeding purees. If finger foods are working better anyway, I'd just stick with them. I probably wouldn't worry about supplements either - for the vitamin D just make sure to get out in the sun a bit every day, though you may want to supp vit D in the winter. And our doctor told me that even though breastmilk isn't high in iron, the iron in breastmilk is very easily digested. Other good sources of iron that my kids love(d) as babies are chickpeas/hummus and raisins/dates. If you're not opposed to it, you can feed him meat too for iron. But really, for DS2 (7.5 months) I just offer him whatever I'm eating as long as it's something he can hold. Sometimes small modifications, like I'll steam baby carrots for him or freeze some of my green smoothie into a popsicle for him.
ETA: Your breastmilk is still the perfect food for him and won't magically stop being so at any certain age. As long as he's nursing enough to meet his caloric needs, he's fine.