Just a few random thoughts, I would be horrified and desperately concerned for his safety if I was in his step/mum's shoes.
What about his job, will they keep it open for him?
I think a travel buddy is a great idea and would insist on it, but first he needs to be much more independent just in case that goes wrong and he's stranded somewhere.
Due to the lack of time before he takes the plunge, why doesn't she give him short sharp lessons on independence, say one every other night, setting him goals and see how he copes.
First, take him to the local bus depot and ask him where he wants to go and how he thinks he can achieve that as opposed to the reality of being able to do it. Show him how to buy a ticket, go with him for the journey, but sit where you can observe him, not next to him, show him how to get off at the right stop in a strange place, then how to find his way to safe overnight accommodation.
Another night, do that at the train station, similar exercise.
I appreciate that this won't be cheap even over short distances but he has to be made aware of the outside world and its pitfalls before he goes off into the unknown with his eyes wide shut.
Commonsense things like not flashing cash or cards or expensive phone.
Drive through some dodgy areas and ask him to give you his opinions and make sure he knows he is likely to come to harm if he's wandering around those on his own. Not all people are approachable, not all people will help.
Lessons on how not to look conspicuously out of place in a strange location will help.
How does he plan on feeding himself and obtaining shelter during this trip?
Is he also planning to travel abroad, because that's a whole other level of things to cope with.
She has to put him in difficult positions, like a strange place, he's been mugged and his phone and money have been stolen, what does he need to do, how can he find help, how can he know offers of help from people who want to exploit him further.
Failing all of that, could his Father come over to where he is and they could do the trip together, make it an adventure that they could share?