Baby ball pythons and juveniles up until the age of about 3 year can eat every 7-10 days. Adult ball pythons (age 3 years and up) can eat every 10-14 days.
You should switch your baby to RATS as soon as you can. Some ball pythons can develop a "taste" for mice and will be harder to switch to rats later on. As an adult, a ball python will eat one rat... if he's still on mice, it'll be 3 or so mice. Which is a pain. So get him on rats as soon as possible.
There is no need to give him a small rat and an adult mouse together. Just get him one appropriately sized rat. If you're financially able, and can afford to buy frozen rats, just throw out the mice (or better yet, give it or sell it to someone who has smaller snakes; use craigslist if you need to).
The best way to keep your ball python healthy is to feed it the right sized prey on a good feeding schedule. Occasionally your ball might decide not to eat (for a week, or two, or a month, or two...) but it's normal. However, feeding them less to stunt their growth is definitely not good for them. He won't get "huge," even if you stick to regular feedings.
If you'd like, you can check out
the feeding page on my website.