I agree with those who say to put him on formula. There are not enough nutrients in just rice cereal and bananas (or any other single ingredients) so you need to make up for decreasing the breast milk by adding something comprehensive. I couldn't breastfeed past 4 months, but we put our child on formula along with a few basic baby foods in small doses and introducing one food at a time. Maybe bananas and rice aren't the best things for your particular child. Maybe he's dehydrated with more food and less liquid too. That's a big factor in intestinal issues and both constipation & diarrhea.
You say the pediatrician gave him antibiotics and that he got well after 2 doses. But did you finish the course of antibiotics even though he seemed better? If not, that was a mistake. If he had a touch of some intestinal bug, you didn't get rid of it in 2 doses. (By the way, everyone else had diarrhea but it was either due to an illness - bacterial or viral, hard to know - or to something like mild food poisoning. It was NOT from the weather change!)
You have a pediatrician - did you consult about food choices, full nourishment and these intestinal changes? It's good that you have keep at least a mental record of how much you gave him over how many days - but do share this with the doctor. A phone call costs you nothing - they will call back if it's not a good time and since this is not an emergency, but i's an important service they provide.
Also, how do you know your child is anxious to try foods? Is he happy to try whatever you offer on the spoon? That's good, of course. But also, he may be putting everything in his mouth, but that's what kids do. I'm not saying he's not interested, but you don't keep giving him things that upset his immature system because you think he's anxious to try them. Focus on him not putting toys or pebbles in his mouth, and don't worry so much about the food. If the doctor suggests that you can try Cheerios, you can do that to satisfy his desire to pick things up and to work on fine motor skills at the same time (just the plain ones, not the ones with lots of sugar added).
Every kid is different, so stop worrying about what you don't remember. What another child did might not be relevant anyway. Try to start fresh and try to find some confidence that you will figure this out in time. You can do this! If your depression doesn't lift, consider possible postpartum issues and not just pinning this on food/digestion. Talk to your own doctor.