Potty Training for Child with Autism

My son with autism is 9.5 years old and is bladder trained with the exception of occasional (once in 8-12 weeks)bed-wetting. He has still not achieved bowel continence, although we have progressed from severe aversion at sitting on toilet to peacefully sitting 2-5 minutes 3-4 times daily. He passes gas when seated on toilet and we praise that as a step toward success as we have other small successes along the way. He is very limited with respect to expressive speech, but can say "potty" and does self-initiate voiding approximately 25% of the time at home. We have been working on this literally for years now and I am beginning to wonder if there is more we should do or if only patience is required. I am receiving pressure from school staff to start using pull-ups or implement some kind of every 30 minute routine which I fear will compromise our gains by making him aversive to toileting. I would welcome any wisdom/experience that you can share. Thanks, Rhonda