If your LO is over 6 months old there really aren't any hard and fast rules about when to introduce most foods, the only 'rules' are regarding things like salt, sugar, honey, whole nuts, other than that it's really whenever you feel like introducing things. If you are concerned about giving too much solids and milk intake reducing then just switch one of the meals you already give and offer the meat, dairy or lentils etc instead. You don't have to stick to fruit veg at every meal or cereal at breakfast. You could also offer the new foods as finger foods (which are advised to be introduced at 6 months) which means LO is getting tastes and exploring the texture and smell etc but possibly not taking in quite as much as he might if you spoon feed it. At a young age it's more about learning different flavours and textures rather than 'eating' a certain amount.
hth