Oh my goodness, he's severely emaciated!! When is the last time he ate or drank? And how much does he eat? He is so skinny, he is dying. I'm sorry but I must be honest with you. It also looks as though the lack of a UVB lamp has caused MBD, which is Metabolic Bone Disease. Bearded dragons MUST HAVE A PROPER UVB LAMP TO PROCESS CALCIUM!! So your dragon is malnourished, dehydrated, emaciated, and has the start of MBD from being under fed and not having a UVB lamp to process his nutrients!!!
He is very, very, very sick, I can't believe your other dragon isn't the same way or close if they're in the same tank!!! Separate them and get this poor guy to a reptile vet ASAP, it doesn't matter what it costs!!!
If you want to have reptiles as pets then you must have the proper setup to keep them alive. You must immediately go and buy a tube UVB lamp and a fixture, a Reptisun 10.0 is best. The fixture is like $10 at Walmart, the bulb will be $30-40 at the pet store. This is not optional, they will both die horribly slow deaths without a tube UVB lamp mounted inside their tank! They need more than warmth! And they also need more than lettuce!!! He needs squash, carrots, collard greens, Dandelion greens, mustard greens, etc. And a bit of fruit maybe once or twice a week. All of this coated in calcium powder and a vitamin powder every day! He needs bathed in warm but not hot water for 15-20 minutes every other day! And he most importantly needs a source of protein...You said he eats crickets, but I'm sorry, that poor emaciated beardie hasn't eaten a cricket in a long time....
He is probably too sick and weak to eat on his own and he needs to go to a reptile specialist asap, like right now, and at this point will probably need to be put in intensive care for tube feedings, otherwise he may need to be euthanized, I'm sorry, but he looks so sick and in so much agony...
I am horribly concerned for your other beardie as well...AND NEVER HOUSE THEM TOGETHER!!!
PLEASE GET THIS POOR LITTLE SOUL HELP IMMEDIATELY!