sick mountain horned lizard

Question
i bought my son a mhd a month ago, he's supposedly 4 or 5 months old. He was doing ok, but yesterday morning his skin looked really dark brown/black. We observed him drinking water, but yesterday i found 8 crickets in his aquarium, i put two in each day, they must have been hiding in the bark.His skin went back to it's normal color thru the day yesterday, now this morning he's turning dark again, he's not eating. I use a blue uvb bulb most of the day and a red one at night. I only give him crickets, but should i be feeding him something else? Please help, i don't want him to die.

Answer
Follow up: RUDE?! I had the decency to not call names as you did, so take from that what you like. I wonder if the poor Mountain Horned Dragon would think me rude for informing you so?  

Everything I said was for the welfare of the animal, and I don't mind that it made you a little miffed over it. My advice is not to make you feel better, it's to care for the animal properly.

You think I sound harsh? What if you were the Mountain Horned Dragon? What then? You really think advocating for the animal is so cruel of me? And it must just be outrageous of me to demand that people buying animals actually do research first, instead of getting their crash course from some minimum wage teenager at a pet store!

Its just common sense that if you care so much and you fear the animal may die...YOU TAKE IT TO THE VET. I don't understand why you people even need to ask questions like that unless you're trying to be cheap and get out of spending money.  
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Your question was sent to the pool for unknown reasons, but I will answer it.

A reptile is not an appropriate pet for a child, and especially not a juvenile MHD, firstly.

The dragon sounds stressed and off temperature, and/or sick, and it sounds like you did not do research on the animal before purchase.

A UVB bulb is not "blue". You need a couple of fluorescent strip lights for this animal which specifically state "UVB" on them, or better yet, a mercury vapor spot lamp, such as a Mega-Ray SB100, or T-Rex Active UV Heat. He is not getting anything with the blue and red incandescent bulbs you are describing.  

You may be feeding improper prey size for this animal's youth, and he may either refuse food, or you could kill him with crickets that are too large through impaction and gastroenteritis. Use age appropriate prey that is no more than 1/2 the length of the lizard's head, and 2/3 the width.

There may be parasites, as no matter where you got him from, there is a chance that he is wild caught, or his parents were. It is still simply too cheap for people to import wild captured rather than spend the money on setups to breed.  

Your temps sound to be off to from the description of his color change. And you did not state what humidity levels you are keeping. All this being moved about, lack of UVB, along with other possible problems, will cause stress; and stress kills reptiles.

If this dragon looks sick to you, and you don't want it to die, then step up and take it to the vet as you should. That's simple enough.

http://www.anapsid.org/treedrag.html
http://www.kingsnake.com/mhd/MHDkingsnake.html