I've been coaching for a lot of years and have worked with some very talented and successful kids, all the way up to the national level. But one of the most rewarding situations I've ever had was an autistic kid who went from not being able or willing to jog a lap as a freshman to running a 3200 in the low 12s as a senior. Not fast of course, but the night he broke 12:30 both he and I were in tears. The four years I worked with him changed my life, and more importantly, changed his.
Number one, work with his parents. Not just talk to them, but work with them. Get them more involved in the process. The normal rules don't apply here. You don't want them to just be supportive and otherwise keep their distance. Trust is key, and he's used to trusting them. They have logged a lot more years with him than you have. Every time my kid raced, his dad was right at trackside on the second turn. The connection was there, and the kid knew it.
Number two, be patient. Incredibly patient. You can't get frustrated. You can't even begin to show frustration. And you can't try to force anything. Keep telling him what to do and what not to do, over and over again, but very calmly and methodically. You never know when the breakthrough might come. But it will come when he's good and ready. Not before.
One specific idea you might try, but only if he's capable of grasping this concept. You and his parents will have to be the judges of that. Have him run some "at pace" 200s in practice. Work with him to get them to the pace you want him to start his 800 with. The idea would be to get him to know what it should feel like to him for the first 200 of the race, regardless of where the competition is. It will take some time, but if you could get him to understand a target effort level for the first 200, that might be a way to get the first part of his race a little more under control.
And of course number three, don't give up. Don't ever give up.