18 year old stepson

Hello all~
First I need to say..what a great forum this is.
I posted a few days ago for the 1st time~ when I was so fed up with my husband & I fighting every day. The input I received was very good. Thank you , one and all.

One thing we argue about is his 18 yr old son (doesn't live with us, but is local and visits often).
He has a "condition" sometimes called retentive encopresis. He has been wearing Depends underwear for about 5 years because he has no bowel control anymore. He holds it for about 4 or 5 days at a time and has been doing that since he was about 10 or 11.

His mother (he lives with now) apparently admitted to beating and berating him for accidents during potty training as a toddler. She's a real piece of work....
SO.... he ha s become literally anal retentive since.
When I married his father he was 11 and I became aware of the problem almost immediately. I took him to a psychologist weekly for about a year until he moved in with his mother.
Needless to say, she doesn't get him any help and has excuses, lies etc to explain his situation.
My husband won't take over and get him to the psych Dr.
My daughter and I think the whole thing is terrible...
He hasn't gotten his permit, has no part time job, failed 10th grade, gets bad grades and plays video games in ALL his spare time. She doesn't get his hair cut, his nails are long, he wears very weird clothing, is immature and naive...
It is very bad influence for his younger brother who stays there on weakends. His younger brother is 13 and has lived with us since he was 6. He does well, but has a difficult time going back and forth between the vastly different home environments...

SO...I don't know what to do. I am the stepmother. Non custodial besides... But his natural parents are clueless...

We fight about him whenever he comes up in conversation and after he leaves a visit here.