6 year old with ASD has toileting anxiety and I don't know what to do

My son is 6 and has high functioning autism. He's a happy, sweet boy and I generally don't have too much trouble caring for him (I'm a single parent), but his toileting issues are becoming a struggle.He holds his bowels for DAYS at a time. He had constipation a while ago and is afraid it'll hurt. The problem is that because he holds on for so long, it ends up being so big that it does hurt when he eventually goes - which only serves to reaffirm his anxiety. Last time he nearly blocked our toilet, it was so bad. He also regularly has to come home from school early for having wet/soiled himself. Sometimes this happens multiple times per day/week. This is hard because I have to work and attend classes at uni and can't always leave in the middle of the day to collect him.I took him to a paediatrician who said that there's no underlying physiological issue. I've also tried an occupational therapist. So far I've:-Increased fibre and water intake -Done rewards charts -Had a regular routine of taking him multiple times per day, at set times. Also encouraged his teachers to do this. -Made the toilet into a "friendly" space with posters of his favourite cartoons on the walls -Tried explaining to him the value of going regularly -Tried incentivising going with iPad or minecraft play - I never get angry or annoyed if he has an accident. I'm always very gentle and understanding to try and minimise his anxietyNothing has worked!! I'm at my wits end. I worry that if he keeps soiling himself at school he's going to get bullied. Does anyone have any advice on other things I could try?tl;dr - my son holds onto his bowels for days at a time due to toilet anxiety and then soils himself. I feel like I've tried everything and nothing helps. What do I do? via /r/Parenting http://ift.tt/2zsXFrF