Yes - technically you are not allowed to sell a mustang without the title passed on from the BLM. You apply to get the title after you have owned the horse for one year.
You can contact the BLM and tell them your horse's freezebrand #, and they can tell you if the horse was titled. If not, you can apply for a title one year from now, but until then, you don't technically own the horse and still have to meet BLM requirements for adoption (as far as I know).
Meeting requirements shouldn't be too hard since he is gentled :) And if he was titled after all and you didn't get that paperwork, then the BLM can send it to you :)
I love mustangs! Congrats!