He can hitch if he has to - I know you care very much for Gabe, but it seems that this may be the wake-up call he needs. It can't be pretty or it won't get through his head.
I think the Big Brother program is an awesome idea! They do things with the kids, too, so you can still get your break occasionally. Contact the school, too - they often have mentor programs that match a corporate person up with kids who can use an outside "ear" during the day to talk, play a board game, get advice, etc. Marc is also involved in karate - can you talk to the teacher to see if he knows an older student or family who might "mentor" Marc, as well? Do I remember him being in Boy Scouts - another place to look. Of Marc's friends, is there a family who would consider letting him go to a sleepover, even once a month?
Do not feel badly about asking for help - that is what it is there for. When my daughter had to have major surgery, I had to search out help from our small town's social service department. The woman there was so understanding, telling me to go to the pantry anytime I needed, set me up with a town-based organization who helped families (they ended up paying for my son's preschool for 3 months, 2 mortgage payments, 3 months electric bill...) Once things were better, I gave back - still do. I cook for families, donate when and what I can, etc.
Also, how is your mom's SS not helping? I hope that she contributes to the bills! Have you contacted the local hospital for caregiver's respite services? You might even be able to have a homecare worker come and give you some "time off," help with housekeeping, etc., a few times a month.
Please, try not to be embarrassed. These are the circumstances you find yourself in, now - it won't be forever. Just be determined to do whatever you need to, to make sure that YOU are ok, so you can take care of your son (most importantly) and your mom (btw, any sibs you can call on, if only for some financial support?)