If hes in keto then taking insulin is counter productive as it suppresses ketone production and also the dosages would have to be small to stop him going hypo
There was a fad of using a little insulin without eating to get into ketosis faster after carb load on a CKD diet and they claimed ketones. showed up much quicker, in a matter of hours Nowadays there's a fad of taking 2iu Humalog on an empty stomach before fasting cardio to burn up the carbs and access fat easier, I think Aceto does it. I doubt it does much of anything considering the overall picture. I would do it if I thought it worked, if I was trying to diet that is lol. It's not dangerous at such low doses for me, in my experience.
About maybe 20 years ago there was an article on how to run insulin while on a zero carb/very high protein diet. High protein isn't keto as mentioned above and I'm sure some could use a good dose of insulin on an excessive protein diet. What bodybuilders do isn't keto since you have to eat 80 something % fat and only little protein.
Some guys who imo are legit smart say the benefits of low carb diet aren't necessarily due to ketone production at all and it's useless to track them.
Just some thoughts