Forget crickets. Crickets are pretty much one of the worst foods for them in terms of getting them to eat them, and they aren't great nutritionally but they are okay.
The easiest thing to get them to eat typically is soft bodied worms. If this one is thin, then I'd go for the following:
Butterworms
Hornworms
Nightcrawlers
Typically butterworms aren't a great food because they are high in fat, so you don't want to make them a staple feeder. Hornworms are high in water and also not a good staple feeder. Nightcrawlers are actually an all around good feeder, but can be difficult and expensive to use on a regular basis and there are concerns about parasites and chemical contamination as most Nightcrawlers are hand picked in the wild from farm lands.
However, these are all great "rehab" foods. Good foods to get a lizard going that hasn't been eating well and they will provide a lot of
hydration which is also typically a problem when they haven't been eating. Silkworms are also great, and a good staple feeder but they can be hard to find and hard to care for, even for short periods.
Once they get going again, dubia roaches are generally the best staple feeder, in addition to collard greens.
You can order all of these here:
http://www.rainbowmealworms.net/
Don't feed more than 2 Horworms a day, but they can eat as many Nightcrawlers as they want. Butter worms should also be 2 or 3 a day at most, down to 1 every other day once they get back to a good weight and feeding habit.