My DS is also 3 and a half and has ASD, I potty trained him a few months ago and he went from resisting to even sit on the potty with his clothes on to being completely toilet trained in less than 2 weeks. And yep, chocolate buttons had a lot to do with it!
I started by making a step-by-step visual guide and posting that on the bathroom door, I think that really helped him understand what was expected of him. You can download some good pictures from do2learn.com/
We then went cold turkey - no more nappies during the day under any circumstances! - and reinforced even the slightest approximations of the desired behaviour with chocolate, praise and a sticker chart. I.e., at first he got the full works for merely sitting on the potty for a few seconds, then only for weeing into the potty standing up (that is, me "catching" it!), then for weeing into the potty sitting down, then for weeing into the potty with his trousers and pants pulled down to the knees rather than being taken off completely, etc.
The first 3 or 4 days were horrendous, first he was weeing and pooing everywhere, then he held his wee in until he was doubled over in pain... I was this close to giving up! And then on day 4 everything just fell into place. He actually started using the big toilet on his own.
I think the crucial thing about any kind of motivator is that you have to be really, really strict about withholding it for anything but the desired behaviour, otherwise it won't work. HTH