My Ryan has ADHD he is 14 now but started goalkeeping around 8 at first it was difficult to get him to try and focus especially when like others have said the game is being played on the other half of the pitch. I started by taking him out and throwing balls and taking shots at him for as long as he could manage to focus and slowly built the time up, it helps no end that he is also now medicated. I have ADHD as well so maybe that helped with the understanding. Now we go out maybe an hour before he has to be at either practice or a match and we run some drills and if there is anything that he wants to practice we do that as well and that gets him in to the correct head space and because he can "get his silliness out" while with me it doesnt tend to interfere with his "match head" It was hard at first as I was trying to do what he was doing at training but soon found that letting him lead the session with me helped a lot. It tires him out but he manages a 70 minute game and only starts to waiver and have a mental reset towards the end of each half. If theres anything you want to ask etc. Just give me a shout.

@Preston Oldfield this is really sound advice thanks for sharing