Weight and a picture will give a better idea if he is healthy at the size he is act.
Dragons can grow at different rates but 8" snout to tail length for a 8 month old dragon is very small. Most are at that size by 8 weeks. If the dragon is a hybrid between an Inland Bearded dragon and a Rankins (or Lawsons) Bearded dragon, that would explain the current size.
Insufficient calcium in your dragon's diet can explain poor growth rate though usually it causes twitching and soft bones and bone deformities long before it reduces growth, so MBD may not be the issue here...unless your dragon has other symptoms you haven't mentioned besides not growing fast. Shaking, difficulty closing the mouth, jaw looking 'rubbery', unable to stand on all legs even when startled (some dragons will 'lounge' on their stomachs, legs splayed out while basking or resting, but if 'startled' or something catches their interest, they should get to their feet and be very alert)
I also find it odd that your cricket supplier will toss in mealworms with your crickets for free...some cricket suppliers use carrion beetle larvae to help clean up dead crickets...but these don't look anything like mealworms..unless Flukers uses mealworms to do this instead (though this is odd as mealworms are not big carrion eaters compared to say, superworms or other beetle larvae). But if you are getting more mealworms than crickets in your order, I would recommend switching cricket suppliers. Mealworms are actually cheaper than crickets and if they are substituting crickets with mealworms, not worth the money you are paying.
So far though, the basking temps are good though its odd he is in a cooler area all the time...what are you using to measure you basking temps with?
Also, how active is your dragon? Does he eat alot? What sort of substrate do you use? What cage size is he in?
You be surprised how a few things 'wrong' in husbandry can affect a dragon's overall health. If he isn't getting enough calcium then that affects his health greatly, including growth. If there isn't enough blood calcium the body ends up leaching it from the bones and eventually the bones get soft, deformed and the dragon can't grow properly, move about, eat etc. Calcium is also used in more than just bone building, it is also needed for other bodily functions, including muscle control, which can cause muscle weakness and twitching...A fast growing baby needs alot of calcium, if he isn't getting enough it will affect the overall health. Crickets and mealworms alone, even well gutloaded don't have a good balance of calcium/phosphorus hence why dusting them with a calcium supplement is important, especially with young dragons who are not eating alot of greens.
Getting a fecal to a vet isn't expensive...most only charge $25-$45 for a fecal and physical exam. Paying for vet care is one of the expenses every pet owner has to be willing to cover when they take on the responsibility of caring for a pet. Doesn't matter if the pet is a $10 lizard or a $5000 purebred dog.
That aside, there really are only about 4 causes for slow or stunted growth. The number one cause of slow growth or failure to thrive is related to Husbandry and care issues. Fortunately this is also what a pet owner has the most control over. But for us to advise, you do have to tell us as much detail as you can, the more the better.
Parasites is possible and would be the second thing I would look into after husbandry issues. After that its either a physiological reason (ie organ failure, glands not functioning properly, tumor interfering with regular function of something in the body etc) or its genetic in nature, such as a hybrid (Inland/Rankins), too much inbreeding or breeding of many generations of small dragons. Genetics can be ruled out fairly easy by finding out the adult size of parents and the current size of your dragon's siblings...and other clutch siblings. Hybrids often have a distinct look to them in pattern etc, which we can identify visually with a good picture. Also other clutch mates will be similar in size.
Parasites and other physiological reason require vet visits, no way getting around it. (or at least no safe way).
-----
PHLdyPayne