Milk is the best nutrition for baby, whether it is breast milk or formula. It is generally recommended that you introduce other food (solid) to baby when he is 6 months old and e solid food is mostly to provide calories for him, as he is getting more active by 6 mths. When you first introduce solid you only give him very little (1-2 tablespoons) and his main meals should still be milk.
IMO, try to stick to milk for now (until he regains his appetite). Was he recently vaccinated? My baby drank less after his vaccination, but after a few days was back to normal. Sometimes baby drink more sometimes less