Definitely trial and error! For me, weight training can actually make my BGs go up. Distance running usually makes me drop, but if I am sprinting or running competitively, I can go up.
When I don't know how a certain activity will affect me, I usually begin with a temporary reduction in my basal rate (about 25%) and about 15g of carbs in the middle of the workout. I check frequently. Then, once I find a regimen that works, I check less, but always make sure to have fast-acting carbs with me.
Different types of exercise, as you know, have different impacts on the BG because depending on what your heart-rate and muscles are doing, changes what your body uses for fuel.
What he's doing is going to be a mix of anaerobic and aerobic, therefore it sounds like he'll actually be using LESS glucose for fuel than he has in x-country and basketball. During x-country and bball, your blood sugar is UP for a long period of time, which causes you to burn a lot of glucose for fuel.
In weight training, the bursts of intensity are very short, and therefore, it allows your body to burn more fat for fuel. When your heart-rate comes down in between sets of strength exercises, your body can get oxygen to the fat cells more easily for fuel. This means he won't have to reduce his insulin doses or eat as many carbs before this type of workout in order to keep his BG safe.
I used to be a competitive powerlifter, and set about 15 records in drug-free powerlifting. I wrote a book, "Your Diabetes Science Experiment" that talks about this physiology at length, available on my website and amazon.
The number one most important thing is that he takes good notes about the carbs he ate, the adjustments in his insulin and the blood sugar results. If he's too high, then he knows he needed less carbs or more insulin. Too low: less insulin or more carbs.
Continue the experiment until he finds his balance. It's all science. It all happens for a reason, we just need to understand the science and adjust accordingly.
Ginger’s book really is brilliant, I highly recommend you give it a read if your son does a lot of sport. It goes into a lot of detail about how various physical activities can affect your BG, and I found it a great help when I first started weight training (and with much else besides).
Thank you for taking the time to write such an awesome book Ginger, it has helped me lead a healthier happier life :)