Your new mustang probably doesn’t know what a carrot is. Nor an apple. Never heard of crimped oats, rice bran or any of the other grains.
So first time you give him these treats he might just walk away. May even pee on them.
If that happens try again in a few days. Eventually he’ll decide he likes them, even waits at the gate while you’re mixing them up in the barn. (Especially if you have a consistent pattern in your chores around the ranch: It’s dusk, my corral has been cleaned, my water buckets are full, I know what that human is doing in there.)
Also remember that he doesn’t know what worm paste is, never had a syringe shoved in his mouth…at least by you. So use this opportunity to teach him to take the medicine in his grain. Smear some paste on a few pieces of carrot, same for the apples. If you feed cookies, put a dab on them too. One third of a tube per horse (this is usually less than the recommended dosage). Repeat tomorrow.
Look at his poop next few days to gauge effectiveness. Follow the manufacturer’s recommendations for intervals between treatments.