Liam hun get him eating wax worms, thats the quickest thing to get the weight on him. I had one that came to me weighing 16g and 2 weeks later was up to 22g from what I remember. Someone told me that you will be able to SEE the weight going on him daily, it starts at the tip of the tail and works its way toward the hips. It's true, I actually COUNTED the kinks in his tail and saw that the bone showed less an less each day. If he is underweight he is going to need worming to ensure he CAN put the weight on. I personally beleive that a few shops dont appreciate that collareds are ravenous eaters, and that the stress of keeping males together can result in rapid weight loss. For the wormer just get him to the vet and ask for some granules or paste, my vet gives me granulated puppy wormer for my collareds. Careful with the mealworms too, try to feed him less mealies, or at least only the white ones that have recently shed their exo-skeleton.
Take some pics when he has settled in, and weigh him/measure him. It always helps to know the length vs weight, post it on here and Andy or I can tell you what you should be aiming for in weight based on his length (in inches pls, lol)
BTW Am i right in thinking he is eating 10 mealies and 5 crickets in total a day? Sounds like he still has an appetite, lol.