He could try lactase tablets prior to consuming dairy products made from cow's milk to find out if they help.
This would confirm the lactose suspicion.
And if no positive results are experienced, an alpha S1 casein allergy from cow's milk is the next possibility to rule out.
Goat's milk does not contain this protein. In addition to this, its fat molecules are smaller than cow's milk & contains less lactose so it is better digested.
It is safe to assume that it's not lactose intolerance, making the possibility of
an alpha S1 casein allergy from cow's milk stronger.
So, yes, I think trying goat's milk is warranted.
FYI here's a simple test that you may find helpful:
Dr. Cocoa's Pulse Test (simplified version)
Just sit in a chair and come to rest for about 5 minutes. Take your pulse for a full 1 minute (not the 15 sec. multiply by 4 thing). Then put whatever you want to test in your mouth and chew it for 30 seconds.
For liquids just swish gently with a tablespoon or two of the liquid in question, instead.
Then take your pulse again. If you find it is faster by 6 beats, you are very likely allergic or have a sensitivity to it. If you have type “O” blood, use 4 beats as the criteria instead of 6. It’s that simple.
I was diagnosed as lactose intolerant and because I cut it out completely, the smallest amount now makes me uncomfortable (I even have mild discomfort using a Lactaid pill). However, because goats and sheep don't have lactose, I am able to enjoy goat and sheep milk dairy with no problems. Also, the lactose is removed in Ghee (clarified butter) and I have been able to cook with it without any problems as well as any lactose-free cow dairy products like Lactaid milk. Also, when I travel, I refer to a website called HappyCow which includes a list of restaurants that accommodate non-dairy and vegan diets (eating vegan when traveling is the safest way to avoid lactose and ensure it doesn't ruin a vacation). Good luck.