Well the first thing is that “bulking” should never be part of the plan, especially if you are an athlete.
I used to coach high school football and always told the players that they are a lot more likely to be one yard short/late than one pound short.\
In other words, SPEED and AGILITY are SO much more important than size, especially in high school. He is 14, let’s say 15. So that gives him at least 4 or 5 year to put mass on his frame. It is much smarter to do it while improving performance than focusing on bulk an end up being slower.
I made the mistake of “bulking up” for football. I was an undersized linebacker (5’9" 190) and bulked up to 225 over the summer (I was drinking at least 8000 calories from shakes per day, having 5-6 burgers per day, etc.). My waist went from 32 to 40. I got stronger, but also a lot slower… I clocked at 5.21 on the forty!!! Lost my starting job after two games because I was too slow.
That having been said the Zombie apocalypse is a good workout for his goal. But it might be too much of a step, for now, going from 2 or 3 sessions a week to that. Might want to do a more traditional workout focused on the basics using an upper/lower split 4 days a week with 1 day of speed, agility and conditioning for a few months than go from there.