Jose,
There is really no calculator to determine a bows cast based on draw weight that would work across the board for all the different types of bows. A general rule of thumb for self bows to determine if they are adequate shooters is that they cast an arrow a distance equal to the draw weight plus 100 yards. So, a 50 lb self bow should shoot an arrow 150 yards. If it is exceptionally well tillered, it might shoot 20 or so yards farther.
Almost any glass backed bow should do this easily and then some with arrows in the 8 to 10 grains per pound range. That is, say if you have a 50 pound bow, 10 grains per pounds is a 500 grain arrow total weight. Eight gr/lb would be a 400 gr. arrow. I did not see any mention of minim arrow weights in the regulations but 8 to 10 gr/in is standard for most trad guys.
By the CA state regulation, that means that on average, depending on bow design and efficiency, about 30 lbs is the minimum draw weight. Of course, it depends on the actual performance of the bow. The kicker might be that if say I wanted to take my kid hunting and she had a 35lb bow, I'd want her to use arrows that were about 10 to 12 gr/in just for the added penetration an heavy arrow provides but if I went too heavy, they might not go 130 yards. I guess to be legal, you'd just have to go out and test your setup. For your son pulling only 26 inches, if he was using a bow 40 lb at 28" the draw weight at his draw would be about 35 lbs. Getting down close to the range where you need to try for distance with his actual hunting arrows.
I'm sure that the "horizontal distance" refers to point A to point B distance measurement taken on the ground between you and where your arrow lands. You have to aim about 35 degrees upwards to get the best distance from a bow shot. My bow that is 65# at 32" will shoot a hunting arrow about 195 to 205 paces.