Tyler went through this stage too. And actually still does toss things off the side of the high chair.
First, give him very small portions at a time. We find that if we put too much food down, ds gets overwhelmed and starts playing with it. You can always give him more.
Also, try not to get upset when he throw something off the side or starts playing with his food. Just say firmly, "We do not throw food, we eat food" if he continues, he might be full so just take him out of his high chair. Do it calmly which I know is difficult.
13 months is still really young to understand what he is doing. He is just discovering trying new stuff out.
Ds is 17 months and FINALLY is starting to hold up his food to hand to me when he is full. Before he would just start throwing so we knew he was full.
Good luck.