Potty training a nonverbal autistic child

My little one is turning 3 and it seems like he is ready to learn to go potty. My issue is I can't get him to go potty on the toilet. He doesn't talk so the communication thing isn't there. The terms they use for him is he is a nonverbal autistic child with severe global developmental delays. He also has SPD (sensory processing disorder). He wont leave his clothes on. I can't seem to find clothes that would be difficult for him to take off. He wont leave his diaper/pull ups on, which is my biggest problem. He knows how to take them off and he is peeing/pooping everywhere. I'm trying to see if I can get suggestions on what would work well for him...

Onesies, full footsie pjs put on backwards (to keep him from getting to the diaper)

Other than that, I'm not much help lol My 5 yr old ASD son brought me a handful of his poop this morning  

I've heard elimination communication is great for special needs children. Look into it. A book I've heard recommended is Diaper Free by Ingrid Bauer. He doesn't need to be able to speak to communicate his needs to you. Good luck with everything!

Do you use PECS? If you have any luck with it that can be really helpful.

My son is talking now, just doesn't bother to let me know if he needs to go, and if I ask (or tell him we are going to try) he runs away and does it in his diaper first. We do have first morning pee on the toilet usually though.

My best friend has a severely autistic (older) sister. They use flash cards and pictures to communicate, as she can't talk either.

He knows how to get out of onsies...no matter what way I put it on him. Ill look into that book. I need to do something. Cleaning poop out of carpets is not easy and he is suppose to start school soon so now Im getting concerned >.>

Right now we are using flash cards and sign language. He was doing well in the beginning, but now he can care less. Hopefully in school they will be able to help him more since it will be 5 days a weekof learning instead of 2 days a week of therapy.