He's headed toward severe Phimosis - the Phimotic Ring - the "elastic band" within the foreskin - is tightening up. There's a cure for that. A quick trip to the doctor for a steroid cream is all that's needed to loosen the band.
It may take some time, but it would be better than circumcision.
If the cream doesn't work, then there is what is known as a "partial circumcision" wherein only the Phimotic Ring itself is removed. This option leaves the foreskin loose, requiring manual manipulation to retract, and to replace over the glans.
However, the immediate issue is that he's not cleaning himself. It's not only gross, he's not doing himself any favors. In the long run what is collecting under there (called Smegma) will eventually begin to have an negative affect on the head of his cock. Tell him to stand in the shower, and use a couple drops of shampoo (or shower gel) and try getting a q-tip in there. He must make sure he rinses thoroughly, though. He doesn't want to leave soap under there any more than he wants the Smegma.
But, the end game...he needs to go see his doctor.
The sooner the better. He'll be glad he did.