Hi T.- Sounds like your son is ready for solids, as many babies are around 4 months. Preferences for which foods to start with vary considerably among moms, along with pediatricians! (Always confusing when we are told it's a mothers choice!!)
Most recommend starting with traditional baby rice cereal, which is the least allergenic food. Mix two teaspoons of rice cereal in a small dish with your son's formula until it's a thick soup. Serve it at the same temperature as the formula you give your son.
After the rice cereal, here's the advice I've gathered from all the Moms I know: Add one pureed vegetable to your son's diet, then a pureed fruit, then add another rice cereal before more fruit and vegetables. Introduce yellow-colored fruit and vegetables first, then orange, then green, based on what most babies seem to prefer. Nothing like starting with bananas and working your way to peas! Favorite first foods: bananas, pears, avocados, peaches, applesauce, squash and sweet potatoes.
Also, introduce one solid food at a time in four-day intervals, so you can tell if your son has an allergic reaction to a particular food. Signs of allergy include gas, diarrhea, rash, hives, puffy eyes, which can show up immediately or up to a couple days later.
Hope this helps!!