There's also a lot of water in some foods (like bananas are 3/4 water, so is yoghurt, most fruits and vegies are more than 3/4 water), so his total amount of drinking will be less than before he was eating. If I add up all the drinking and bf-ing that my 18 month old does it's probably 500 ml or so - but he seems happy and healthy... I wouldn't worry about the fluids unless you see signs of lack of fluid (e.g. hard poo). Keep offering water pretty often though.
I would stick to the BF first most days and not worry if he doesn't want brekky. He's starting to self wean, in the sense that he's not automatically going to want a boob unless he's hungry or thirsty or tired or wanting a cuddle or..... But finishing self weaning is probably a long time off!