giving the size he appears in the picture where he is lying across your finger, he seems to about 6-8" long (depending on how long your finger is). After three months, he should be well over 10" so he definitely is not growing as he should.
Getting a fecal done is a very good ideal, as he could have parasites.
If he is getting tired of crickets or showing very little interest in them, try small silkworms, butterworms, the new phoenix worms (these are very wiggly and my dragon can't get enough of them)
Feed as much as he will eat in a 5-10 minute period. Try to feed 2-3 times a day, spread out as much as possible (ie feeding in the morning, around lunch time, and early evening, at least 2 hours before lights go out for the night).
I would also get rid of the sand. Sand is very dangerous to small dragons as it can cause impactions very easily in them. I am not sure what horicultural sand is, if it is coarse, very fine, etc. Basic paper towel is the best substrate for babies. It is much easier to clean and won't be accidently eaten by your dragon.
Young dragons don't take to greens as well as older dragons but it should always be offered. You can try offering it by hand to your dragon, dropping small crickets on top of the greens (or other feeder insects) which can burrow into the greens or the greens may just be taken in at the same time a the insect. For small dragons you don't need tuns of salad in the bowl, a pinch of finely chopped greens (maybe a teaspoon or tablespoon, well misted with water) is enough. The dragon may pick at it during the day but if you offer too much, there is no real way to tell if he is eating any.
You didn't mention (or I just forgot) if you are supplementing with calcium and multi-vitamin powders. It is best to dust your insects once a day for young dragons to ensure they get all their vitamins and calcium for their fast growing bodies. Also, what are you using to measure you temperatures? Not all thermometers are very accurate, especially the 'tape' or dial styles sold in most petstores. These can be off by as much as 15F. A digital thermometer with probe or temp gun is much more accurate. You can also lay the probe directly on the basking spot to get an ideal of the temperature.
Make sure your dragon is not dehydrated, mist him with water daily and provide a shadow water dish (though most dragon's won't drink from a water dish some will). A weekly bath in luke warm water for 10-15 minutes often encourages your dragon to drink.
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PHLdyPayne