It's not always a good idea when you first start out on BnB to take long term guests. If you don't like them for whatever reason or there are problems - it's a more difficult situation.
And those living in your accomodation full time will be around and want to use your household facilities much more than a temporary visitor.
Is this what you are looking for?
It's against BnB rules for someone to book for someone else, so if you want to consider the booking then she can ask her son to register himself. You can set your profile settings so he has to be verified ie provide passport/driving license/social media etc.
If he's doing a placement at uni - setting up his own profile shouldn't be too onerous.
If I had a long term guest I would ask questions around what they like and do ie will they be home all the time or out and about. Will they expect to have their friends around, hang out in your living room and cook meals in your kitchen.
I think with long term guests people will change bedding weekly or provide so guests can do this, themselves.