A few things that spring to mind:
If he is usually on his phone at home then is he connected to your wifi? If so then you may be able to contact your ISP for details of web search history, downloaded info etc on his phone via your internet. Not sure how tricky this can be but the data is there in the ether somewhere if he has been on the wifi. Obviously it won’t pick up anything that he searched while connected to 4G or other wifi but it may be a start.
have you tried guessing his passwords for email addresses etc? If he has booked anything then confirmations would need to be sent to an email address.
Have you mapped out the train route between your location and Luton and thought about where Luton may be en route to?
Is the bank able to tell you which ATMs have been accessed ie where? Do these line up with the Luton sighting? Do they imply he is still in Luton or that he stopped in Luton en route to somewhere else?
Thinking more about possible motivations for leaving. You said he was due to move out next week. Why was he moving? Has he lived on his own before/not at home? Was he due to live alone or with a friend? Had he already passed the referencing checks?
i agree with pp that you need to speak to his dad. He could even possibly be involved? Or have some relevant info? They may have been in contact and you not know about it. Could have been a trigger?
Work. How did he get the job he currently has? Sometimes people at work know more about someone’s lifestyle than their other friends. Have you spoken to people he hung out with at work, after work? Could there have been anyone he owed money to? The deposit was presumably a big sum of money so if he had that then he wouldn’t have needed to access his bank so soon after if he was just wandering around Luton. He may have used the deposit money to a) book a flight, from Luton, although it’s very difficult (not impossible) to get on a plane without a passport or b) pay off a debt. If it turns out to be the latter then what could the debt be for? Any involvement in drugs or anything else? Gambling, even online? Did he have a credit card? People with ADHD can be more vulnerable to addiction issues.
Im assuming you’ve scoured his room etc. Have you found anything unusual or unexpected? Was it more or less clean than usual? Did you go through any rubbish from the time he left, find any receipts, any trace of him having been doing anything out of the ordinary?
Finally (don’t answer on here if it needs to be kept private) but could the incident about the petrol be at all relevant? Do you know who did it or suspect who did it? Did your son know them? Did he know about what happened? If not, could he have found out about the incident and it triggered something? Sorry for all the questions they’re mainly just prompts of things to think about rather than stuff you have to answer on here! Wishing you all the very best.