Hey ZoeCat,
Hope you and your lil man are well! I definitely agree with Marijke, stop the rice cereal for now and start on the veggies. I was told to try all the orange veggies first. If you really not keen on making your own baby food I would suggest Hipp and Olli organic baby food, so maybe give those brands a try.
To help now for the constipation, my paed said prune puree or treacle brown sugar but what really worked well for Ali was Treacle brown sugar (make sure its not plain brown sugar as it wont work) 1 1/2 teaspoon in her formula (150ml back then), ive also had her on Reuterina drops (5 drops in her food).
Hope that helps