starting solids at 4 months

Sorry this is a long one. My son is 4 months old and I feel like he might be ready to start some solids. he eats 6oz per feeding totalling 30oz a day and he is starting to cry after finishing most feedings but I know he doesn't have gas. he's done this before when I was giving him 5oz a feeding so I increased to 6oz and he was satisfied. now he is starting to do it again and I'm thinking it might be time to start some cereal/solids since I know they shouldn't have more then 32oz in a day. he has amazing head control, opens his mouth when anything goes near his mouth and tries to put everything in his mouth, loves to see what people are eating and always wants to be involved at the dinner table. he can't quite sit up totally on his own he just needs a little bit of help or to sit in his bumbo and he's fine. he has a checkup in a little over a week so I will definitely talk to his doctor about starting food but I'd like to get some advice from other moms on when and how they started out with real food/baby cereal. I know they recommend starting at 6 months but it seems like he is almost ready now, and I've heard it's okay to start around 4 months if they seem ready and have doubled their birth weight which he has. Any advice would be appreciated!