If you give your child straight whole milk without introducing it to his tummy graudually it can really cause issues you don't want to deal with.
I had one lady who's WIC messed up, it stopped formula a month early and gave her whole milk instead. She had no money to buy more formula and she went cold turkey to whole milk. Of course it didn't hurt the child in the long run, she just had the worst smelling poo of any child, in any child care setting, of any child in the whole wide world...lol... I have worked child care a lot of my adult life and poopy diapers don't bother me. These diapers had me changing her with a trash can handy. Her poo stank so bad the kids in the other classrooms were gagging too.
It was from the milk not getting introduced slowly. I would always tell parents take it slow and introduce 2 oz. per week. Stat with 6 oz. formula and 2 oz, whole milk. Then go to 50/50. Then a week or so later go 60% whole milk and add 2oz formula. If he does this well he can switch as soon as you think his tummy is fully ready.