David, I too don't see the 5 month span as making a great impact on his future at this point but we could both be wrong. The big downside is the expense. If you have no problem with the expense then even if there was little to gain there is no big problem. If your son is focused on improving, at least he has established a starting point with the feedback from PG.
If it were me at this point: (it's not but for sake of discussion)
Before you read anything I say: I'm not suggesting you "push" your son, I'm making suggestions to how you can "help" your son.
I would buy a stop watch and help him establish a workout. I would monitor his speed progress at least once a weekly and document that progress and share that with him.
"A goal without a plan is a dream". I think I had that pasted on the wall beside my son's soft toss machine.
I would determine now what position he is going to showcase at and focus on that position. While he can showcase at two positions, he needs to determine his primary position before he starts trying to showcase. Develop specific drills for that position -- For instance if it's the outfield then the mechanics of the crow hop greatly impacts the velocity of the throw from the outfield.
Arm strength: Always important. Again develop a specific program. Long toss is by far the most popular. Establish the # of throws, the distances etc. At 78 mph it looks good but improvements are great.
His batting: Plate discipline is a plus in a game situation but doesn't garner attention at a showcase. I would spend the showcase money on private hitting lessons. Develop proper mechanics and then he should POUND the ball daily with either soft toss or "T" drills. Set up a net. If your son wants to be the best he can be ---- and he doesn't have callouses on his hands ---- he needs do more.
Remember it's not really about showcasing, it's about being the best you can be. Showcasing allows you to display what you have.
Fungo