Inkanta - ask yourself why the man probably felt safer asking for the child's seat than asking any of the adults to move. As so many have said on this thread, it is probably because he has, on many other occasions, asked adults to move and been abused for it. And this abuse is being heaped on top of all the other struggles that his disability causes him.
Given all of that, I can understand why he asked for the child's seat - why he made the choice not to risk yet another mouthful of abuse from an adult. Put yourself in his shoes, and ask yourself whether, in his place, you would have happily laid yourself open to verbal and even physical abuse for asking to use a Priority seat.
The sad thing is that it didn't work. This poor man still got abused.
In an ideal world, one of the adults in the priority seats would have offered the chap with the disability a seat. In a slightly less ideal world, one of them would have got up when he asked for the child's seat. Or someone would have offered the OP and her child a seat, when her child got up, to give the man his seat. In an ideal world, disabled people would not get physically and verbally abused for wanting to use the Priority seating on public transport. People would understand that this seating is not a perk - it is something that makes things you and I take for granted, possible for someone with a disability - ^and that without these things, people with disabilities would not be able to lead what most people think of as a normal life - going to work, to the shops, to medical appointments, to visit friends and family.
These things are not luxuries - they are aids that make life possible for people. A priority seat on the bus is no different to a crutch, a walking stick, a guide dog or a wheelchair. If you would happily take away a blind person's guide dog, or tip someone out of their wheelchair, then by all means sit in the Priority seating and refuse to give it up when someone who needs it, someone for whom it is intended, asks to use it. And by all means, abuse them for asking.
Most people would not take away the crutch, wheelchair or guide dog, or give a person abuse for needing to use them - so why is Priority seating different?