Off season HS pitcher workouts?

Squats and deadlifts.  Med balls for rotational work.  Work in long toss 6 weeks out from spring practice.

What strength training experience does your son have?  That might help to make the suggestions more suitable.

What has he been doing thus far during his 2 days a week in the gym.  What's his plan if he ramps up to 3?

2018 Starting a winter velocity throwing program at Advanced Baseball in Burke VA (Dec - Feb).  Designed to maximize performance while minimizing risk.  Squats & deadlifts on primary days, and light shoulder work with bands, and rowing on off days. Lots of food, MetRx, and TwinLabs Amino Fuel.

I have to be honest.  I have reached paralysis by analysis on this one.  I am NOT an expert on the strength and fitness thing so I read and I read and I read.  And seems like every 'expert' has a slightly different take.  I have given up.  He is doing football workout and adding some baseball specific core work to it.  Also long toss and yoga to at least attempt to get more flexible.  Maybe it will pay dividends and maybe it won't.  But I think if you are just fit and strong however you get there it is a plus.

I agree with 2020DAD. I would just have him go with the driveline program and get a workout buddy to help in understanding their instructions, plus he has the HS coach to help. Getting bigger stronger faster is going to improve himself regardless what he is doing, just get going and he will learn.  IMO the main injury risk is doing Olympic style lifts without some training. If he is in the gym then I would have him focus on lower body and core and sequence a workout that goes from mass and strength building now up until January and then move over to explosive focus.  Get someone who can help him with technique particularly for Deadlifts and cleans. 

I only have around 35-40 mins in the weight room with my HS baseball players and we try to get a good mix in during the offseason. Start with a good dynamic warmup/active stretch. Usually go with 4 main and auxiliary lifts along with some med ball or ab work, 3 times a week. Really like trap bar squats, goblet squats, lunge variations (regular, reverse, lateral, single leg, and walking), RDL's, and step ups for main lower body lifts and throw in some plyo stuff to add some variation (box jumps, box 2 box, frog jumps, skiiers). I think that those allow for strength, power, and mobility in there. Upper body wise we do a lot of push-up variations (traditional, weighted, elevated, A frame, shoulder touch), pull-ups, inverted rows, upright rows, bent over db rows, and I'm sure I'm forgetting some. We always static stretch/foam roll at the end too. 

Meant to put that I like most of those lifts because they aren't highly technical lifts and form can be taught fairly simply. We have already been lifting/plyocare/long toss and will shut down throwing after this week until we come back in January. Once we shut down throwing we will lower the reps and up the weight to really hit a good strength phase on some of the movements until we get back from Christmas Break. 

CMASSRHPDAD,

If you're in central Mass or really anywhere around Boston, Cressey Performance in Hudson is a great facility for workouts.  Lots of pro, college, and hs guys there.  Lots of pitchers.  My oldest is there 6x a week. And many guys getting their throwing program in too.

My son is working with a trainer who specializes in youth athletes. Honestly I don't know exactly what they are doing because I have not watched him. He drives himself there.

Branson, I have looked into Cressey. Are you really paying $729/month for your son to be there 6x / week? That's a tuition payment. I can't swing that.

He is right on the bubble in terms of where he needs to be for his stretch baseball schools.
His pitching coach is clocking him at 84-85 in the cages, but outdoors at camps and showcases he is 80-83. I don't think coach is blowing smoke. I think maybe my son is taking some off for control when he throws live.

He has told me pretty adamantly that he does not want to throw weighted baseballs. He does not think it's good for his arm. This is supposed to be the time for him to shut his arm down and get in the gym, but some big camps are coming up starting mid-late December. I want to do everything I can to help him but he needs to want it too. Can't buy him 87mph.