@Thankfuckitsfriday1, ignore the haters, especially those who don't seem to have read or understood your posts.
I think @Imchlibob's suggestion to contact the bus company to get them on board is a great one. I would suggest something along the following lines based on my understanding of the facts.
Dear sir/madam,
I am writing to enquire about the arrangements on your buses concerning disabled passengers, and to request reasonable adjustments to your service for my disabled son.
My son, [name of Thankfuckitsfriday1's son], has autism. Please find enclosed with this letter medical evidence of his diagnosis. Due to his autism, he can find certain situations distressing because they are overwhelming. In particular, when travelling on buses he needs to remain seated on the buggy board attached to the buggy which my daughter also uses. When I have attempted to move him to the bus seats, it has resulted in a meltdown caused by his autism with him on the floor crying, kicking and unable to express or understand why he cannot sit in the one place he feels safe outside the house. I should emphasise that these episodes are a result of his autism, are extreme and are not the same as a tantrum which a neurotypical child may have. [Amend this as needed so that it is accurate. Add any further detail - in particular, if you feel these meltdowns may result in harm to your DS or others it would be helpful to say this]
From September, I will need to take my son to pre-school on the bus and back 3 times a week. As above, he will need to remain seated for these journeys on the buggy board attached to the buggy which my daughter sits in, and I plan to place the buggy in the wheelchair space. Unlike most parents who use the wheelchair space for their children's buggies, I will unfortunately be unable to fold my away my buggy and move my children to the bus seats if a wheelchair user wishes to board the bus due to my son's need to remain in the buggy seat. I believe that this is unlikely to cause a problem for the vast majority of journeys because my understanding is that the bus is safely able to accommodate one wheelchair and one buggy simultaneously. However, I am concerned about what may happen if my buggy and another buggy are occupying the wheelchair space and a wheelchair user wishes to board the bus.
Please could you confirm what your company's policy is regarding the use of wheelchair spaces on the buses?
As you may be aware, in FirstGroup plc v Paulley [2017] UKSC 4, the Supreme Court confirmed that there was no requirement for bus companies to have an absolute rule that wheelchair spaces must be vacated if a wheelchair user wishes to board the bus. Further, as my son is disabled for the purposes of the Equality Act 2010 you are under a duty to make reasonable adjustments to the service provided to him.
I believe that it would be reasonable for you to allow my buggy to remain in the wheelchair space even if other parents are asked to fold up and move their buggies to allow a wheelchair user to board the bus. As mentioned above, I believe the buses can safely accommodate my buggy together with a wheelchair user so this should not impede the wheelchair user's journey.
Further, if the wheelchair space is fully occupied by non-disabled buggy users, I believe that it would be reasonable for the bus driver to request that one or more of those passengers should fold away their buggies so that my disabled child can use the wheelchair space and remain seated on the buggy board. [You haven't mentioned this but I imagine it's a problem that could arise and you should really get priority in such a situation over non-disabled users]
I would be most grateful if you could confirm whether you are willing to make the above requested reasonable adjustments in accordance with your duty under the Equality Act. It would be extremely helpful if you could provide a brief letter stating the adjustments that you are willing to make which I would be able to show to your bus drivers if necessary.
I would be very happy to explain my son's needs further if you wish to discuss or require any clarification of the information I have provided.
Yours faithfully,
[Thankfuckitsfriday1]
Attached: [eg letter from specialist confirming autism diagnosis]