It's correct that a baby might not poop for 5-6 days. Especially as their diet is getting changes. However, at 6 months old, it's not necessarily appropriate to start messing with water (as their electrolytes can get dangerously messed up), I wouldn't start adding things like prunes, since he's only eating a tiny bit of solid. Other than perhaps a 1/8 teaspoon puree (talk with pedi) in the cereal mix.
Regardless, I would be making a call to the pedi on day 3 to discuss WHEN you should be worried.
When you start adding other foods, consider ones that assist with smooth movements... anything with a pit... Like perhaps avocado. Then peas, (fiber), carrots... etc...
Banana's are constipating.
Although a child is probably not in any danger if they don't poop daily, they are not functioning at optimum. They should have a big healthy poop DAILY. I'm talking as they get fully introduced to solids. I have 4 y/o's. They poop daily, and it looks like something out of an adult... and has for over a 2 years. HUGE! Daily! One eats more fruits and veges than the other, and you can tell. The quality, quantity, and how often one poops is indicative of a good diet.