I would do milk first and then solids about an hour later ... any reason why you're doing solids first and then milk? Solids is just for fun in the first 12 months and yes, I would cut back on solids too. I didn't do solids first until around 12mo with the girls and as I mentioned before, with dd3 (9.5mo), I am still offering her milk first and then solids later and if she doesn't have that much, I don't push her and I just end the meal and move onto more activity.
Yes, too much could be causing discomfort... perhaps he's too distracted during the day. If it were me, I wouldn't do baby cereal at all. As I mentioned earlier, that stuff caused lots of constipation in my lo and I could tell she was uncomfortable so I stopped it altogether. She still doesn't have it. They don't have to have a certain amount of tea/tablespoons (although there is a guide to how much) but as I mentioned earlier, I believe in self regulation and if mine wants to eat what I give her, she picks it up and eats it and if not, I'll take her out and we end the meal.
Have you heard of baby lead weaning?