I hate to say it but if he's MSPI and your milk has dairy/soy in it, he won't be able to tolerate it. If you know aother family who could use it who hasn't got dairy/soy issues that's one possible outlet for it, but otherwise.... sorry.
I know, it's like liquid gold!
)
You're probably fine with all meats and pretty much all veg in your diet, along with most good fats, like coconut oil, olive oil, avocadoes, fish oil, and possibly even butter (although you want to be off dairy completely for a month to be sure - 2 weeks to clear your system and 2 more to clear his); butter often has so little milk proteins in it that it's OK for many who have to avoid most dairy, but better to clear it completely and then do a separate trial to be sure. But if he's so sensitive that the little you're giving him is affecting his poops that much, I'd avoid the dairy and soy utterly and completely.
Oh, and for family who give you grief, I found that asking them how much they spent on formula in a week, let alone a month or a year, often shut up a lot of people.