At the college level right now our team typically lifts 3 days a week. Monday which is an off day from baseball is typically a heavier day total body, Wednesday is heavy-ish total body, Thursday has a pretty heavy power focus and super low reps. My understanding is that in order to maintain the gains doing high weight for very low reps works best while limiting soreness (My roommates and I have found this to be true). The low weight high reps personally makes me feel pretty sore and at the end of the season my strength numbers had dropped a decent amount, making it feel like a bit of a waste.
The thought process is basically that high reps is pretty taxing on CNS, and also tears more muscle tissue making it harder to recover. I kind of view it similarly to lactic acid, running long distance actually causes it because of the muscle breakdown, potentially making you worse off than doing nothing for post throw recovery (Something Zach Dechant has touched on on twitter).
Also when I say low reps high weight, that is typically geared toward lower body. For example, in the off-season I may work up to a weight on deadlifts and work there for 5 sets of 3 reps, in-season this would fall on my Monday and look like working up to a top weight and going for 3 sets of 2 reps. So I still get the load of working up to top weight, and put my body under that top weight tension, without doing so much that I am going to be sore the next day.
I know Tread was mentioned in another post, and when I worked with them this was also pretty similar to how they worked, although it was during summer ball so it was a bit more aggressive lifting as the stakes are a bit lower than during the college season.
As a two-way it may be tough, but finding at least a day or two a week to get some sort of workout in would be huge. Hopefully your son isn't playing ss the day after pitching, which might make the night of the start or next morning a good time to get a heavier lift in with low reps to keep the gains while also being lower impact on the body. Trying to get a lift in within 24 hours of pitching (or playing really) is ideal too as it puts all the stress in one cycle and allows your body to start recovering earlier. Depending on how many games your son plays a week, the 2nd day could be a power/movement/mobility focus with jumps and med ball stuff, working on being explosive and getting work in on the left side of his body to help prevent him developing imbalances in his body.
Hope this helps