Welcome to Purrs, you are in the best place for advice here!!
Biscuits like Whiskas and supermarket brands are known to make cats sick - they have a high cereal content which some cats can't tolerate. You might want to look into things like James Wellbeloved, HiLife, Hills, Royal Canin, Burns - they are more expensive, but you do feed less, and they have rice rather than cereal, which is easier on the cats stomach, and they dont have all hte colourings etc that some of the supermarket brands have.
The dandruff may just be his skin adapting to being in a home environment with central heating etc, the majority of cats that come here get it initially but it soon dies down. There are supplements you can get for that though, I just tend to give it some time first.
As for your toast, no tips there I'm afraid, none of mine do it!!