In my opinion both stretching and strengthening are in order. No need to hang him upside from the porch just yet. I have a good strengthening exercise that you can do without any equipment if you are willing to help him out as resistance.
Here is how to do it:
Each sit on a chair facing in towards each other. Your kitchen chairs should work quite well for that. Have him hold his arms straight out towards you with elbows locked and clasp his hands together. It looks like the arm position for a bump in volleyball only the elbows are straight.
Your job is to push his arms to the side (alternating sides) while he resists. He should resist until he is facing almost directly to the side and feeling the stretch in his side abs and lower back.
At that point hold that position while he continues to resist then remove your hands in an instant and let him snap back to the center. This mimics the explosive movement of the golf swing. Repeat the same thing to the other side to balance the muscle groups.
Part of the problem with golfing is that you only swing from one side. This leads to a muscle imbalance which can lead to the pain you are talking about. By working both sides, you can correct that imbalance and reduce the pain.
Do between 5 to 7 reps on each side (alternating sides as you go) for 3 sets. Take about a minute rest between each set. This can be done 3 to 4 times a week, e.g. Mon, Wed, Fri and Sat or Sunday.
Try to increase resistance as he gets stronger with the movement.
Another very similar variation you can try is to make it a competition. Start in the exact same position. He will do everything he can to keep his arms pointing straight forward at you while you try your hardest to push his arms to the sides like above.
Don't worry about resistance or getting the stretch or anything like that. The elastic force he will get from trying to keep his arms forward will work great. Use fast, sudden movements from either side without warning.
Naturally, don't start with this one. Let him develop some core strength for a few weeks before trying it.
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