It doesn't sound like he was transitioned. I may be wrong, sorry if I am.
When ready to move from formula to regular milk start the first week by adding 2 oz. of regular milk to 6 oz's of formula. Do this for several days up to a week or until he is really, really doing well. I never had anyone longer than a week but some might take more time.
The next week go to half milk and half formula. Do this every bottle, every day, for about another week. If he is doing well and likes it start the next week by going to 6 oz.'s regular milk to 2 oz.'s of formula. He should be able to go to regular milk full time after a couple of days of this.
It takes about a month to slowly transition a baby to regular milk. It can be done cold turkey but the pooh is the nastiest, grossest smelling, make you throw up if you even are in the same room kind of pooh. One of my kids in child care center got WIC and the mom forgot to start transitioning the baby girl the month she turned one, she thought they had another month of formula coming and they were planning on doing it the next month. They had no money and got several gallons of regular milk so they had no choice but to go cold turkey. I finally couldn't stand the pooh anymore and bought a can of formula and we got the baby's tummy back on track and transitioned her right and the nasty pooh was gone by the second or third day. It was so bad I puked over a trash can while changing the baby's diaper. I had never even retched while changing a diaper before in 13 years.
If you check with your local Health Department about getting some fliers regarding toddler nutrition you'll see that toddlers need the fat in whole milk for proper development. It is important to follow the nutrition guidelines so your baby will have the best nutrition possible.