I'm trying to add a feature to my follower where, if the player mounts a horse, the follower will summon forth their horse, and follow the player.

Summoning the horse when the player mounts a horse works fine, but the follower refuses to mount his summoned horse. The horse can be ridden, and behaves just fine.

How would one get my follower to ride his horse?

There's a mod that does this. http://skyrim.nexusmods.com/mods/14037/?tab=2&navtag=%2Fajax%2Fmodfiles%2F%3Fid%3D14037&pUp=1I believe.

*edit*

disclaimer for +18 mods being on Nexus

I'm afraid that doesn't help me, all the scripts lack a source for me to examine.

Managed to figure it out.