He'll have a regular "Wanna Get Away" fare or else one of the other standard adult tickets, so no worry there and feel free to book him online as far as I know you will be able to enter the required infomration online when you book:
His mom should ask for a "gate pass" to stay with him through the origination airport through boarding of the aircraft and hang out until the plane takes off.
You should do likewise in order to meet him at the gate at the destination.
As far as connections he's older than the age where you can pay extra for official Southwest assistance. If possible obviously a direct flight would be the best option, even if it costs a little extra due to demand. ($50 is what you would have paid for escort of a <12 year old, so on that order of incremental cost should be no-brainer to go direct.)
The passport would be helpful to have for demonstrating age although photo ID isn't required, proof of age is required for Southwest. TSA doesn't require photo ID for minors.
Home airport MDW, frequent visitor to MCO to see the mouse.