are you talking about leopard geckos? if so the vivarium you have will be fine for him and the females when they are adults. however, if he is still young and on his own the viv is in my opinion a bit on the big side. if the gecko is in too big a space he'll be stressed out and will be reluctant to eat also he might just not be able to find his food. i would transfer him to a smaller viv (24x24x12) until hes about 10 months old. as you will prob know, leos are nocturnal so he'll be eating most of his food when your asleep. to ease your concern, my leopard gecko hardly eats any of her crickets but shes still a little fatty..basically dont worry too much until you know for certain whats wrong.
i read your other message and the walking backwards with his mouth open sounds like he is stressed. have you much hides and decor in the viv? fill the viv with cork bark and rocks and logs, a small amount of greenery would be beneficial but not too much as you dont want to create the wrong habitat. oh and one more thing check the temps and your set up. a 2%uvb light on for 12 hours a day will indicate the daytime for your gecko and will bring out his bright colours. dont use spot bulbs for geckos use heat mats connected to a mat-stat at one end of the viv to create a hot end. also invest in a digital thermometer for total accuracy. the temperature at the hot end at night should be about 90 degrees F (remember this will be just on the floor where the heat mat is, the middle should be 80 and the cool end should be 70. place the probe of the thermostat on the floor of the viv where the heatmat is. always put heat mats under the viv.hopefully if everything is correct hell eat like a horse! good luck!