I've reached a point where I want to back off and feel that I don't want hitch my wagon to a guy if I'm going to get an extra passenger. I'm concerned this kid will be mooching indefinitely.
In a nutshell, he treats this kid like he was 14 and he acts like he's 14 too. He's a kind kid, and is loyal, has a good heart, but what good is that when he can't function in society without daddy?
Since some boomer women here do have some adult children living at home/returning to live at home temporarily (which I'm not in that situation), maybe more meaningful info. before we give our opinion at length:
Presumably the child appears to be mooching because: he is not working / contributing abit of money to household expenses; does not do some household chores??
Kid's personality sounds like a good base (ie. no drugs,violence, anger management problems at this time).
Based on what happened to me and siblings when we were at that tender age, all of us were generally hardworking teens that grew into university kids who all held down a part-time job/summer job to help supplement our educational grant tuition and other costs. We were probably your model teens/young adults (with all sort of normal fits of rebellious anger).
We knew our hard our parents worked to sacrifice their dreams for us. They were and still are low-income level group.
Yet, dear mother still had to gently ask us after we graduated from university, got our first jobs, for abit of money to pay some household expenses where we lived in a house of theirs. They STILL had to ask us to get off our lazy butts to cut the lawn, etc. and other base chores to fulfill our parents' absentee landlord responsibilties.
A child needs to transition to adulthood by having the parent ask..now to help out with household chores, etc. You need to chat up with your BF about this and it's best he as the father, provide direction to son. You're just GF to the son.
The ball is in your court if you choose to live under the same roof with BF (if you are) or live closeby elsewhere while still have a great relationship with BF.
This will take time for son to change and also for father-son relationship to grow poistively without undue dependency.