This is a decision your gona have to make. Understand this up front 20% is not enough for him to continue to make repairs and make his payments. This is another one of the facts of life of rushing into an investment without proper capitol now he is just gona be happy to get this monkey off his back.
It also spells that he is a poor business decision maker to have his back against a wall like this. Now how can you capitolize? If it is a decent truck and you want to continue forward, than I would proceed as it is a good deal for a smart business person. Just two words of caution get everything in writing including dates of how long your gona stay in this truck. Cause the downside I see is you help him and out and in a few more months he finds another driver at a 60-40 split and your history, so cover your rear. As this without it spelled out in writing could get ugly.
Second, just be prepared at this revenue that if he cannot fix a repair you better be ready to do so,and willing cause remember its your business and your getting a great deal here. He aint makin squat. So for you to get peeved cause he can't fix it just remember he is already broke, and two broke people aint gona get a truck running, and if he does not make those payments your truck is gona go away. Make sure also that you see somehow that those payments are not getting behind and are being made or than again its gona go away. Ask how is he gona be able to afford repairs (I do not see he that he will be able to even if he wants to) also how is the truck gona be insured, registered etc.. this all costs $ and from what I see he has none by your above question. Hope this helps wish you both well.