Struggling to coach my son

Hi there, 

I'm looking for advise, I'm my sons football coach (U11 low-mid ability team). And we are a mixed ability team and win some/lose some. My ethos is, as long as the boys are enjoying playing with their mates then all is good and there is always another game... next week. 

However I really struggle with my own son, technically he is very good, he undertakes additional coaching once a week and is football mad. He has shown me he can be really good. However, when it comes to our team on a sunday, he makes no effort and doesn't apply himself. 

I will obviously play a part in this as I can get very frustrated with him, I have reflected and I will make sure a treat him the same as everyone else going forward (less commands or shouting). 

But how can I get him to apply himself. He isn't very big, nor fast but is technically very good. He plays both striker and goal keeper. And is very good at both, but he just won't run, or assert himself. 

Any ideas? Or am I being unreasonable? 

Thanks

M