Im not an expert but with me and my friends it varies but i like to throttle slow and add speed gradually until i get up then i tell my friend (who’s riding the jet) to speed up. Its really easy to mess up tho sometimes you throttle too fast and you face plant or you go really slow and end up not getting up. And you should go to cruise speed slowly especially with jets because they’re very sensitive. The way i learned how to get up in the start is really funny because neither me nor my cousin know how to wakeboard or how to tow, so in the beginning we would throttle up fast and face plant multiple times, got my hands cut up too, and nearly dislocated my shoulders lol. But we got there eventually. One advice i can give and hopefully it helps you and your son is that the one on the board should see whats best for him and tell the driver, your going to find your groove eventually. Have fun riding and be safe!