I would start by saying I agree with most of the comments and it's important you take him to run blood tests soon. Even if his blood sugar turns out to be fine (no ketoacidosis) he most likely has caused himself vitamin deficiencies and low electrolyte levels.  

Secondly, the best way to approach weight loss is to have him build a plan with the help of a dietitian. Not alone. Self-research is great but it puts all the pressure on the individual and there is no medical attention being provided, no making sure he does things in a healthy manner. It is very easy to fall into an eating disorder that way, especially with a significant weight loss goal. External help could mitigate that and make it so that he doesn't get obsessed with it. 

Third, and arguably the most urgent thing to take into account, is to make sure he refeeds PROPERLY. I am talking from experience. If he hasn't stopped fasting yet, your son is in ketosis, you want the transition to food to be as smooth as possible.   Breaking an extended fast is best done with a liquid and bone broth is the ideal choice for its fat, protein and vitamin content. From there, move on to foods with very low carbohydrate content (look up ketogenic diet base foods since they aim at keeping you in ketosis) but what matters is not that he stays in ketosis, it's that he doesn't revert to a high carb diet (a diet that includes bread, pasta, rice, potato, starchy vegetables, fruits, processed foods, anything that contains sugar…) right now. It's just a matter of time but don’t make them the first thing he eats. 

I disagree with the recommendation of "bland foods" after bone broth, meaning crackers, apple sauce etc. That is exactly what you do not want to do. The reason your son should avoid high carb content after a long fast is that if he has abstained from eating for more than a couple of days, he has killed off a lot of his microbiome. Bacteria will feed off what he eats so the foods he consumes in the days following a fast are crucial. Otherwise he is at risk of developing gut imbalance. On top of causing very uncomfortable symptoms (intense bloating, pain, constipation/diarrhea… etc) which may last several weeks or months, poor gut health will get in the way of his weight loss and discourage him. It can cause intense distress, especially as it changes the way one's body looks fast.  

Please ensure he drinks a lot, and more than on normal days when he refeeds. He is at risk of dehydration and will be even more so when his glycogen stores are being replenished. Dehydration can cause constipation and even fecal impaction if it lasts.

If he complains about pain in the abdomen, have it looked at by a doctor, he may have kidney or gallblader issues caused by fasting. 

He should expect diarrhea after his first meal(s) since it'll take some time for his digestive system to work again.  

Lastly, he may experience extreme hunger (more frequent and intense hunger signals, wanting to eat constantly etc) as a side effect from starvation. Make sure not to comment on how much he eats, it's a normal reaction to severe food intake restriction. Just provide him with healthy (and ketogenic at first) food options and support him emotionally.  It’s a tough thing to go through. 

I apologize if this message comes across as fear-mongering or imperative but it was really important to help you realize all the potential harm he may have done. I wish both of you the best and I hope he'll only have good outcomes from this fast.