When you feed the baby hold him too. I know this is super difficult, I did it with my two who are 20 months apart. The defiance lessened because I was spending super special time with the two of them.
Also kids at that age are curious. They do things because they can. What we see as defiance isn't defiance at all, just them being curious. The bigger of reaction they get from you the bigger they think they are. So if you get up set and put him in time out he sees that he has power over you. Instead of letting him form your reactions figure out what to do to make it where he can't throw his plate off the high chair. Get him a big-boy table. Let him eat his food directly off the high chair without a plate instead of being set in your ways and thinking it has to be a certain way.
If your child doesn't assert his independence and show that he can act independently then when he is older he won't know how to. As he grows you will want him to be more independent but if he never was allowed to go through the independent stage and was labeled as a defiant child, then when he is a teen he will be defiant. It is counter intuitive to what almost all parent "experts" say, but it's the truth. Squashing a child's independence at around 2, labeling them as defiant makes them defiant teens.