Not sure if you resolved your problem but wanted to chime in. DS also has SPD and did not like the Nautilus, he couldn't tell me why just "no no no like seat".
He also one day began complaining about his XTSL hurting his back - I couldn't ever find anything that could be causing the problem. I had to switch it to my husband's car out of mine (he rides in DH's car for much shorter periods of time) because every time we went every where it started a tantrum.
I find that with DS once he has made up his mind or declared something to be so, it doesn't matter if it's "true" or not. He isn't budging from that first feeling/thought - more so than neurotypical children (because let's be honest, all kids and adults can be stubborn
)
For example, he needed bigger boots so I bought him the stupid expensive Spiderman snow boots, in the correct size. He loved them in the store. First time out at school he tripped in them and declared them "no no big" and would not put them back on his feet. Even after he outgrew them, he still insisted that they were too big. Perhaps I should have "forced" him to deal with it, but a 45 minute screaming fit every day just was more than I could handle. I should have returned them right away and got my money back.
So my :twocents: is if you can, return it.