we were in the exact same situation a few months ago, probably the same age as Finn now or Adam's age. Like Laura and Yazzie said, baby tools just don't WORK! especially the fork! And DS just wanted ours. Once DH put his fork down, he reached out and had his tiny meltdown and i always had to tell "no" otherwise DH would hand it over.
What I tried:
1) told DS that I was just trying to help him and only hold the end of the spoon (to stablise it), so it got into the mouth and then cheered;
2) filled a second spoon like pp's
3) occasionally offered some spoonfuls from my plate (which is the same food)
anyways... using yoghurt to practise with the spoon is really great! Fork still doesn't get to DS's hand in this place yet

And suddenly about a month ago, he could feed himself completely almost without spilling, (except the times things just wouldn't stay on the spoon.)
It's just a few weeks of practice, u will be amazed as to how well he has mastered the skills!

Also, try giving him spoon in bath or with playdoh or with sand, just keep practising