well honestly it does NOT have to be a van...
it is just that a van is so EASY to make accessible.
You have some advantages since he is young/light weight and isn't driving.
The travel trailer also means you can setup a base camp and then day trip away from that in your tow rig.
Here are some "off the top of my head" choices ranging from simple to more complex.
4wd van...super simple, plain & easy
It can either be a full size or midsize or even the older toyotas
4wd extended cab truck with 60% of back seat removed for chair storage. There are some cool arms that will come out & lift the chair after you have placed your son in a child seat. I say extended cab over crew cab as you can open both doors and really access the interior.
If you don't want to use up the inside space for the chair then look at the Bruno shell that lifts up out of the way and an arm that brings the chair up into the bed.
4wd ambulance...yeah that is right, just add some more windows and you are good to go
I know I am not giving you a link to just one perfect rig, and that is because this is a process that needs to be explored.
And of course budget is an issue we haven't really touched on.
cigar smoking, wilderness first responding, ham talking night nurse who is overland certified and a fellow of the Royal Geographical Society.....
now everyone say "so what where have you been lately?"