Nephew seems ready but don't know what to do?

My nephew is almost 4 and has been diagnosed with SPD. He goes to pre-k during the day and they say he sits on potty but won't go and very leery about being wiped. We have known this since he a baby.

He also had to have his boy area (not sure if can put the correct term here or not) surgically fixed when 6 mos. so doctors have said he might have phantom pains and it might be harder to train. So we expected it, but he won't even sit on our toilet at home. We have different kinds of potty chairs and seats but he doesn't like any of them.

How do we go about picking a seat or what will work for him?

He seems to have these issues from your article:

- fear sitting on the toilet due to poor muscle tone, postural instability, or poor balance

- be unable to get undressed and get on/off the potty by themselves due to poor sensory-motor issues and coordination

- may gag or become nauseated by the smell of bowel movements or the bathroom in general, thus avoiding or fearing it

- be afraid or fearful of the sound of the toilet flushing

- be uncomfortable on a hard, cold toilet seat

- be uncomfortable with the feeling of rubbing toilet paper on themselves after voiding/eliminating

How do we help him with these? His parents know of his issues but they seem to want him trained now and I want to help him and them, how do I do that?

Because out of your article list on readiness he seems ready except for the sitting on toilet (at least at home) and the pulling own pants down and things like that.