We were so excited all of us getting ready to go out today but DS (autistic) would not go in it. He was so scared or angry (not sure which) that he made himself sick 
He loved the seat before it was attached to the bike and has been out in a bike trailer and enjoys it. Maybe that was the crux of it, that we were getting him to do something unexpected, or maybe he was thinking that perhaps he was expected to stay on the bike by himself.
It's so hard when you have to guess and can't explain, or reason with a child.
We've now shown him some you-tube videos of children and babies using the seats, and tomorrow DH is going to give one of DS' toys an outing in it
for him to see.
The bike DH has now isn't suitable for a trailer and it has to be him pulling it or meltdown so if nothing works I don't know what we're going to do. DS can't or won't ride a bike or trike.
Does anyone else have any experience of this, an insight I might not have thought of, or tips? Thank you.