With my daughter I just switched to whole milk. Gave her a cup of milk. She drank it. She was fine. I was also still breastfeeding. It was fine.
Kids don't have to be "weaned" off of anything to go onto whole milk... UNLESS they are rejecting the taste of whole milk.
Then, you simply add about 1 ounce of whole milk, to the existing thing your child is drinking (ie: breastmilk or Formula), then GRADUALLY, increase the ratios over time. Until the child gets used to it.
That is what I did with my son, who did not immediately "like" the taste of whole milk. I used Formula for him as a Toddler, because he was EXTREMELY picky with eating and per our Pediatrician, it was fine to continue Formula/milk in addition to... his usual foods etc.
He got nutrition that way.
You, do not have to give your son Toddler Formula "for a few months then whole milk", because, he does not seem to have a problem with eating or with drinking.
The milk, has to be WHOLE milk. Because, it has the milk 'fats' in it which is an important nutrient for brain growth.