Kim,
I'd suggest looking at the South Ole Spain Tours web site, and then also email Emily (who owns the company) to inquire about a private journey for you and your son.....and then also compare the price for any group tours to Gibraltar her company might offer from a convenient location. She can even arrange your overnight stays, if you need help w/ that.
Check out reviews for her company on Trip Advisor.....that's how we found her.
My husband and I were in Spain this past spring, and we arranged transportation thru Emily, and it also included a stop in Gilbraltar (my husband really wanted to see Gilbraltar). Emily arranged the most wonderful private guide, and we were able to see everything we wanted to see efficiently and in a very well-informed way: cable car to the top, siege tunnels, the cave, the apes, the lighthouse at the point, and bit of the city......our guide made the few hours extra special. We actually traveled from Granada that day, with a stop in Ronda for lunch and some sightseeing, then Gibraltar, then on to our overnight in the old town section of Marbella. It was a long day, but a fun day, too.
And, I should add, that I was not super excited about going to Gibraltar....I did it for my husband, who was excited like you son. I actually wound up enjoying it and finding it interesting.......the excellent guide helped, I'm sure.
And, having the South Ole Spain Tours driver hand us off to the guide at the border made for a much more efficient crossing....we didn't need to worry about parking or finding our way (they both had cell phones). Traffic can be a zoo entering Gibraltar and/or departing, depending on time of day. Our crossing was seamless and quick.