For a tegu not to eat that much there may be a little problem.
Not eating for a day or two is common, but they typically will acclimate, especially to food. The first thing is to check his temps and make sure they are okay. 110-120 basking spot, 85 degrees for mid-cage areas and 75 or so for the cool area. This allows their body to regulate body temps. Without this regulation, their digestive systems do not function properly.
Another aspect could be parasites. Tegus, especially CB are generally hardy and can deal w/ mild parasites, though do not rule this out! It would be wise anywise to get a stool sample checked out by a vet who is experienced with reptiles/lizards.
One last thing is to try different food. What was he eating in the petshop? The zoomed tegu food is okay, but most do not like this very much. I would suggest to boil chicken and cut it up into pieces that is appropirate for his size. Make sure to dust the chicken w/ a Calcium/Vit D3 supplement. Most tegus love chicken and should trigger his appetite. The San Diego Zoo Diet is terrific as well and most tegus love it. Especially the fruit in it is needed by reds to help them with shedding. you can find the recipe on the old tegu forum here on kingsnake.
Bonemeal maybe difficult to find in health stores any more, so you may subsititue herpivite (1 tbs per lb of meat) & the same measurements for calicum/vit D3 supplement as well.
Good luck!!!
Greg