I am mid-20's. I have noticed that there are two clear streams of men.
Those who are lazy, and nothing motivates them to get or keep a job. They talk a big game - daydream about expensive watches or nice cars or weddings and children and holidays...but they don't want anything enough to actually get a keep a job.
And those who are motivated, or at least have some stamina, and have jobs. They work, every day, and if they lose the job for whatever reason, they are straight back out there getting another one.
Your DP is clearly in the first category. I'd hate door-to-door sales too, but lets face it, if you keep losing jobs and being unemployed, your potential pool of jobs is going to weaken. If he'd stuck it out and done sales for a few months, he could have then started applying to better jobs that he'd prefer.
It's not just his career that he's lazy about, either. He doesn't bother to maintain a social life, he doesn't entertain himself if you're not around, he just hangs around his relative's house waiting for you. And not even that - living with a relative and not being able to do anything - is enough to make him think, I'll get a job and get my own place/be able to afford nice hobbies.
You can't change him. Infact, you're probably stopping him from changing, because subconciously he's now ticked the girlfriend box and a few times a week he gets to live with you and think "Hey, I've done alright not having a job".
Whether it's too late to have a family is up to you to decide. Whether it's too late to save this guy already is - he can't borrow your motivation, it'll never last and he'll resent you for making him work. If you can even get him to work in the first place - and so far he managed 2 months, out of twelve.