Sam,
You raise a number of interesting issues.

1) Your role. You are both a coach and an athlete. I think you have realised how difficult that is. As an athlete you value your own performance, but as a coach you must value the performance of your 'charges' above all else. This is source of conflict that you might be able to manage, for example by doing your training at other times. If you can't, it might be worth considering which is more important to you, coaching or performing?

2) It would appear that your athlete defines 'success' in terms of matching your performance. He needs to be able to define success in his own terms. There are a number of things that you might consider here.
First of all, you could use performance profiling (devised by Richard Butler) to help draw out what he thinks are important skills and attributes, and to help him become more aware of his own performance. One important point about profoling is that it is based on Personal Construct Theory - it is important that the skills and attributes are his concepts, not yours or those out by theory.

To do this ask him to think about all of the things that he needs to be able to do to be a great thrower. If he can think of a role model, then he might be able to describe the qualitites that he sees in them. If he chooses you, that might not be so bad, but you can ask "why me?", and have a discussion about that. The idea here might not be to change his mind, but to help him see that there might be a wider world out there with better examples and higher standards that he can aim for. Anyway, take his list of attributes and make sure that he can put a brief definition, in his own words, to them.

You can then take each attribute and ask him to score himself out of ten. As his coach you might also score him and compare/negotiate. Then, take tht score from the ideal score for each (strictly speaking the athlete can set this at any number from 1-10, but usually I just set this at 10 because most youg athletes do and find it confusing otherwise).

Next, ask him to rate the importance of each attribute out of 10.

Finally, work out his final score which = (ideal - current) x importance.

This gives a clear indication of his priorities for development.

Then, instead of just setting goals in terms of throwing distance, which can always be compared to yours, you can help him to develop goals around his own priorities for development.

Something else to consider is that you are a major influence on this boy, and your behaviour will have an impact on his beliefs about success. By demostrating by throwing, as another athlete as well as a coach, you are envouraging him to compare himself to you. Imagine that you were not a strong thrower yourself but were his coach. You'd still have to demonstrate to him, but would have to show good technique without the end result of a long throw. How can you do this yourself? Maybe you can try harder to isolate parts of the technique when demonstrating, and not release - I don't know. What you can certainly do is encourage effort and improvement, and model this yourself. If you compare yourself to other athletes yourself you will be modelling the beliefs that you are trying to discourage...

I hope this is helpful.
Rob

Thanx for the reply

I think a performance profile would be useful, hadnt thought of that

Here is an address to the coaches throw association:

Check with Rob he may have a resource or an answer to the question you posed.