What I get out of what you wrote is that you agree with the philosophy of letting your baby lead you in what he is ready for.
Your doctor is not concerned about your son's weight or food intake, probably because your son has always been in the 3rd percentile or within 5-15 percentile points of the 3rd percentile - is that correct? Doctors aren't concerned with percentiles as long as there isn't a significant drop or gain of 15 or so points. If your son has always been in or around the 3rd percentile, your doctor is right to not be concerned.
Your concern is that your baby was sleeping an eight hour stretch, but that has changed to a 3 hour, 4 hour, then 3 hour stretch. Your mom and sister have you worried that the frequent waking is due to your son's lack of eating solid foods.
You can read Help! My Child Won't Eat! published by La Leche League, which will help reassure you that your instincts about following your son's lead with what and how much he wants to eat are correct. You can call a La Leche leader or attend a meeting to learn about the sleep habits of breastfed babies compared to formula fed babies.
All babies are different. Some will sleep longer and more consistently than others. Some will eat more at an earlier age than others. Your baby was sleeping better. Many things can disrupt a baby's sleep: teeth, developmental milestones like crawling, walking, talking, or anything newly learned, a higher need for comfort, etc.
My son never slept long or consistently, and at 7 months began waking every hour. This phase lasted for about 2 months, then got better again, meaning he's sleep for 2-3 hour stretches again. He'll still have a night here and there with the every hour thing. It always coincides with something developmental. He finally started doing a 6 hour stretch fairly consistently a few months ago: at 20 months. My son didn't latch on to eating solid foods with a vengeance until he was 14 months old: he has food allergies and is very texture sensitive, but this was normal for him. Every baby is different. You can try to force something to come, like eating or sleeping, or you can let your baby lead. Find some support for your mothering choices and educate yourself on babies' eating and sleeping habits.