Nursing four times/day doesn't seem like enough to me for a baby who's not eating much solids. What about adding in another nursing session around 5 pm? (It would take a few days before you start making much milk at that time, so you would have to try it for at least five days to know if it will help.)
Or, since he's seven months old, maybe you need to be focusing on upping the solids more? I know you can't make him eat if he doesn't want it, but how are you trying to feed him? Are you doing finger foods, spoon foods, both? Self-feeding, or you feed? Or maybe he's not hungry since you only offer solids right after nursing. I know some recommend always nursing right before solids at this age, but maybe you could experiment with waiting an hour or so after nursing to give solids and see if he eats more.
Has he always woken twice during the night, or is this new? I think one night feed is normal at least until solids are established, but two feeds at 7 months seems unnecessary to me.