OP I think you were probably the safest person for him to approach.
You say the other people looked able-bodied and were young, he may have been wary of their reaction to him. The other person was a man with a baby, and probably wouldn't have been a good choice to ask as he turned out to be a bit aggressive sounding anyway.
In a week when a fifty-year old woman with MS can be punched in the face by a 20-something year old able-bodied man over a parking space, and have her jaw broken in four places with just one punch, I don't blame anyone for approaching the person who looks the least likely to verbally abuse or physically attack them.
My lovely friend, who is in her sixties, who has problems with her back, who suffers from vertigo and fibromyalgia, had her car damaged when she parked in a bay for disabled people, displaying her blue badge, because some idiot passing by decided she wasn't disabled enough as she wasn't in a wheelchair.
Another friend gets regular abuse when she goes out on her mobility scooter. People don't want her on the path, they don't want her on the road, they don't want her in their way in shops, they seem to think that entering the supermarket magically gives her the ability to walk again. She gets told to stay at home, to get out of the fucking way, to speed up, to slow down. No matter what she's doing, someone thinks she should be doing it somewhere else, as far away from them as possible. She gets told she's too young to be disabled, called too fat and lazy to walk, accused of faking her disability for the money.
People are aggressive and rude to people with disabilities, they face this sort of thing on a daily basis, and as my friends both say, they don't know who's going to turn on them or for what.
I agree with you that smaller children are probably safer sitting down, I'd let my son sit down while I stood too. I'm steadier on my feet than he is. But I think you probably appeared to be the safest person to ask on the bus and he was probably upset that the man with the baby started swearing at him and questioning him, and then you joined in by challenging him about his reasons for asking you.
He will get that sort of thing all the time, and I feel a bit sorry for him really. At best he could have done a more general approach to asking everyone if they could let him have a seat, but I'm prepared to bet that he would be ignored at least half the time for doing that. People have a remarkable habit of not hearing or seeing the person doing the asking if they aren't directly approached.