L.A.
It is age appropriate at this point. You put a tiny bit on his plate and you feed him from his fork or spoon. He is still exploring the textures, his fine motor skills and obviously his throwing abilities.
You might give him 2 tortellini on his plate and then actually feed him half of one. If he eats it, then you offer him another half.. If he makes it into his own mouth great.
2 green beans on his plate next and you feed him 1..
2 pieces of melon on the plate, you feed him 1.
Also sounds like he is not really hungry. Children's appetites will ebb and flow. It will feel like they are not eating anything for a few weeks and the next thing you know, they cannot get enough.
You need to follow his lead.
I know it is very frustrating. You are doing fine, reminding him that the food stays on his plate, but in reality, he may not totally understand it. Everything else he is allowed to touch he is allowed to play with it.. Fling it, flip it.. etc. Remember he is only 18 months old. He has not been on this earth very long and most of it has been spent sleeping and exploring.. Nothing will change about this for a while still.