As a follow-up to this comment
I've been struggling to find a bike rear seat for my 5yr old autistic son. He's 20kg right now and I feel he'll outgrown it soon.
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If your child is able to, I'd suggest a "third wheel" style, where you attach a second full seat and a third wheel behind your bike with a pedal and everything. "Trail-a-bike" is one brand, but there are others. This worked well for me with my five year old who was learning to ride but also wanted to go on longer trips with me; he was probably in that ballpark size-wise at the time.
This does require them being able to sit on the seat and stay there, and not be too unpredictable with their motions - if they are too unpredictable, they might make you fall over - and at least the ones I've used don't have any sort of buckles, so they have to be capable of choosing to stay on the bike. So, this will be appropriate for some, but not all, autistic children.
Much of this though depends on your child. I have a friend with a moderately autistic child, and around the same age was able to learn to ride - and in fact loves it, it's their favorite thing to do basically. If both riding for himself, and the trail-a-bike, is too much, then you may want to consider a bike trailer. Bike trailers often are built for two children, and while generally the "per child" weight is less (like 15-25 kg), I suspect you can find one that would work for a somewhat larger child (though with some limits, still.)
Buggys for larger children exist, some of these can be used as bike trailers - this is an example, I don't know the product and can't say anything positive or negative about it. The page might give you useful search terms.
The crucial difference to "ordinary needs" bike trailers is the larger size/weight these can handle. Having a buggy for a larger child may be a beneficial too.
These things are expensive! You should inquire if your health or other insurance can cover at least part of the cost.