are you sure they are fleas? i had a problem very similar to this and it was mites from wood chips i bought for my snakes.
only thing to do is destroy subsrate start again!!, put all furnishings in hot water and soak over night will drown any mites. there is a treatment you can buy that you dilute in water which you bath animal in made for reptiles(but can't remember name) kills eggs aswell. soak furnishings in this aswell.
if poss bath tegus in warm water now to try and drown mites that are on him, then bath again when you get treatment.
just for now wash tank/viv with warm water untill you get treatment.
was crap but snakes had to have newspaper sheets in bottom of tank, do this with your tegus cos you can change sheets every day.(last few days we used plain white a4 so you could see if there where any mites left). do you have something new that he can hid in/under if he need. will take a few days maybe week to get rid of infestation!!
if you follow these steps you should get rid of problen. also tell store where you got substrate from cos it turned out my guy had doddgy batch, (sounds silly but some people buy chipping from garden centre this is big no no)