This is a familiar problem for many coaches of this age group and above.

Firstly, it sounds like you are making many of the more informed decisions on improvement:

1. Equal time for all the players: They learn so much from playing in real games.
2. Encouragement: Plenty of educational evidence to suggest that positive reinforcement leads to intrinsic motivation and better technique.
3. Emphasis on correct technique: This will build confidence in the player and he will learn from his successes.

Your goal setting has pros and cons. They provide a focus, but may lead to disappointment if they are not achieved; and, while achievable goals provide something to build on, if a player is making no tackles, even one is a massive goal for them.

For me, goal setting, at this age, is not really something they take on board as much as we might as adults.

You are also considering making them a focal point against a weaker team. 

Research, looking at young tennis players, suggests this can be demotivational, as well. 

This is because the players might think: ”You are suggesting this lot are easier to tackle – what happens if I can’t even tackle them?".

In my experience, learning to tackle at this age does not conform to normal educational and coaching standards. 

I would suggest the following approach:

1. Take away the emphasis on the player to ’make’ tackles in games.
2. Let his peer group lead the encouragement for him to make tackles, but don’t let them goad him into tackling.
3. Be patient with the three points that you do in training.
4. Be patient with the player and still let him have equal game time.
5. Start the game with the player, rather than have him come on when the others have their minds warmed up.

And this is what really worked for the boys I have coached: lots of low level rough-and-tumble games in training, where there is no hiding place. Often, boys who don’t have older brothers or sisters are not used to this. 

Overall, there is no quick fix. Sometimes a boy will suddenly make the change.