Hi B.!
I am sorry you have had some negative experience with this. It can be frustrating when others try to tell you how to take care of your own baby. I am sure you are doing your best.
My experience with my girls, helped me realize a few thing.
First off, if you trust your doctor to help you with your experience with raising your kids, then I believe they are a good doctor myself. I had a doctor that was good, but something didn't sit right with me about her and her advice, so I went with my gut feeling and found someone new, when the opportunity arised. Anyway, I guess what I am trying to say is that I know that if you didn't feel right about what your doctor was saying, you would look for someone different and obviously you don't need to. :)
The other thing I wanted to mention, was that in my experience even the best of doctors, can't tell a mom what is best for her child. If what they are telling you sounds good and you try it, but for some reason it just doesn't work with your "good feelings" or with your baby, do what you think is best. My doctor told me I could start feeding my babies the cereal pretty young too. My mom fed all of us cereal to "help us sleep through the night" at 8 weeks, and we are all fine. I started to feed my first daughter the cereal at 4 months, and something just didn't feel right to me about it. She was acting more fussy, and she had mucus in her number 2 diapers, which is a sign of allergies, and she was already sleeping through the night without it. So I held off until she was 6 months, and she was fine when I started her then. I had the same experience with my other daughter too and waited until 6 months also. My daughters were both close to 9 lbs when they were born, and they are both tall for their ages too still. Sometimes their tummy's just aren't ready for even rice, the most mild food you can find, and I found out later that the mucus may have just been from her tummy being to sensitive to handle solid foods at the time, and that usually their tummy's are more ready for it around 5-6 months anyway.
It don't think you are feeding him to much personally. As long as he is not gaining more weight than he should and he acts hungary when you offer it to him, his body knows what it wants and needs and you will either see physical signs or feel it intuitionally whether it is more than enough or not. If he eats it, and he doesn't show any signs of sensitivity to it, and you listen to your motherly intuition(which is more real than what others tell you) on what you feel is best, I am sure you are doing great!
I personally don't feel that baby's need the solid foods until they are older, but I also know (from experience again)that if they aren't introduced before 9 months they will have a harder time with eating them, and the earlier you get them used to them the better. I actually watched a little boy who was eating similarly to your little boy, and he is 2 now and very healthy. :)
Good luck, and take everyones advice with a grain of salt, because although they may be telling you something out of "trying to show they care", only you will know whats best for your kids in the end.