My daughter is 6 months old, too (and I have an older daughter who is 2 1/2), and my 6-month-old has started being kind of picky, too, lately. I'm thinking it is partially due to just the fact that she is getting older and having more of an opinion. She used to love the cereal, and I would usually give her cereal and a veggie/fruit at lunch, and the same at dinner, but lately all she wants is the veggie or fruit, and after every bite of cereal she kind of whines a little. So I have to switch between the two (I've tried mixing the two, but she doesn't like that). But anyway, I think some babies just start to develop preferences. I remember my oldest daughter went through times where she loved certain foods, and then didn't like them for a while, and then liked them again. I think you do just have to keep having them try the foods, and if they don't like it, try it again another day. My oldest hated broccoli forever, but I would give her a bite or two every so often, and now she loves it.
Also, on the amount, my understanding is that at this age, they should only be eating about half of a jar of each food. Especially if you're increasing from what you were giving him before, he is likely going to need some time to adjust to the increase in food.
Another possibility is that I know around 6 months they go through a growth spurt, and I've heard (and remember with my first) that sometimes after a growth spurt they have kind of a decrease in appetite for a few days. It could be that, too.
Or it could just be the lingering feelings from the sickness.
I guess the point is: there are a lot of things it could be, but I wouldn't worry too much...he'll eat if he's hungry, so just keep offering it, and so long as he's taking his milk, I've heard that is the most important thing at this age. Then if it continues for very long, you might call his doctor and see what he/she says.
Hope that helps.