If you are really insistent on giving solids how about Avocado? At least avocado has got plenty of good fats in it. I was told that it's the closest food to breastmilk, of course it's nowhere near as good but if you must give food then maybe Avo mashed and mixed with breastmilk or formula is a good option otherwise you'll be replacing the goodness of milk for veggies or rice cereal that don't have very many calories in them and will be filling him but not giving him much nutritionally!