Perhaps since there's some confusion you could be a bit more specific on what his feeding program has been like over the time you've had him.
How long on pinky mice and how many per week?
How long on fuzzy mice and how many per week?
When did he move to adult mice?
Number of mice per week over what time period?
As far as the fecal, call around and find a herp vet that will do a fecal float test for parasites. It can often be done while you wait and doesn't include bringing the snake in to the vet at that time. It usually costs around $30.00.
Check daily and when you find a defecation, scoop it up, pop it into a ziploc baggie and store in your fridge (not freezer). Take it to the vet as soon as you can. If your snake does have internal parasites, do a search here and you'll find threads on what medical treatment is usually done or start a thread. It's best to know as much as you can about that stuff.
It may also be a lifetime of underfeeding (don't skip the fecal float though). One of our females here that we adopted came to us as very undersized. At 18 months she was only 1" longer and the same weight as our 6 month old female. While there's normal variance, Kyna was obviously stunted looking. She had been underfed on overly small mice because her owners were given bad advice by a pet store.
Kyna's feeding program was changed but slowly with no massive prey jumps, just a nice slow progression up in prey size and total volume each week. Eventually she did go to rats from mice. She's now a very normal sized adult female and is in fact in her first breeding year. :)