Hi there!
I sympathize with your trouble in getting important nutritional information from your doctor; the dirty little secret about doctors is that, while they know a lot about diseases and injuries and medications, most know next to nothing about nutrition. I know some folks consult dieticians and nutritionists or even a good book on the subject to get information they need in this area.
I don't think 5 months is late to start on solids at all, in fact my understanding is that solids should not be introduced before 6 months, so as to minimize the risk of creating food allergies. Also, waiting until at least 6 months - when the baby is interested and their intestines are more mature - will probably make the whole process of introducing solids easier and more comfortable for you and your baby.
Years ago when doctors were pushing formula as "best", it became the "fashion" to introduce solids as early as possible, but this practice led to lots of food reactions, allergies and discomfort in the babies. There are no health benefits to feeding cereal to infants younger than 6 months. So don't feel pressured to start giving your son solids until he is really physically ready and at low risk for painful gassy reactions. The later you wait to introduce solids, the more successful (and less risky) the experience will probably be for you both.
At 6 months he'll probably enjoy a little rice cereal mixed with breast milk, fed to him with a spoon (not in a bottle). You can supplement nursing with anywhere from 1 to 3 servings of rice and milk throughout the day, depending upon his interest and appetite. Definitely keep nursing him though, as this will remain his primary source of nutrition. Initially, the rice cereal is really a snack/supplement, to give him the iron which he will no longer be getting from your milk once he gets to 6 months.
After a few weeks or so when he is comfortable with regular rice cereal meals, you can start trying oatmeal, and if that goes well, alternate that with the rice. At 8-9 months, you can start certain fruits and veggies pureed (bananas, winter squash, carrots) but I would check with a nutritionist or baby food cook book (one good one is called "Mommy Made") or other source about timing for introducing certain foods, especially if there is any family history of allergies. Any time that he gets very gassy or cries and is in pain an hour after eating something, that's a sign to hold off on adding that food to his diet until later. And be prepared, once your baby starts eating food, his poop will start looking and smelling like "real" poop! Nothing wrong with that, of course, but it shocks some first-time Moms who have become accustomed to nursing baby poop!!
Good luck and Bon Appetit!